Punctuation behavior in titles and headings.
The 12-hour clock cycles from 1 through 12 and normally needs an AM/PM marker or enough context to distinguish morning from afternoon/evening. Human-facing style guides differ on punctuation, capitalization, and whether to write noon or midnight instead of 12:00 AM/PM.
ISO date-and-time representation uses a 24-hour clock for standardized interchange. A form such as 18:00 avoids the a.m./p.m. distinction, but punctuation, seconds, time-zone information, and human-facing presentation still depend on the applicable standard or house style.
A.M. and AM are formatting alternatives, not different meanings. Chicago documents several legitimate forms and prefers lowercase “a.m.” in its own style, while other publications may choose uppercase or periodless forms.
Identify the noun/pronoun that anchors the phrase, then check that the construction is grammatically separate from the main clause and supplies background such as time, cause, condition, or accompanying circumstance. Do not label every introductory phrase “absolute.”
AKA/aka abbreviates “also known as” and introduces another name for a person or thing. Cambridge’s dictionary and Business English entries present the form without internal periods, so punctuation and capitalization should follow the chosen style rather than an invented universal rule.
AM and PM distinguish the two halves of a 12-hour day, but punctuation and capitalization are style choices. Microsoft house style uses uppercase “AM” and “PM,” preceded by a space; other style guides may use forms such as “a.m.” and “p.m.”
The ampersand is the character “&”; the word “and” is ordinary conjunction text. Microsoft house style generally prefers “and” in prose and headings except where an ampersand belongs to a name, interface label, or technical expression, illustrating that the choice is contextual rather than universal.
An ampersand is meaningful text, not merely decorative punctuation. Chicago normally limits ampersands in edited prose but preserves them in established terms, corporate names, verbatim quotations, and work titles that use the symbol.
Apostrophe decisions are easiest when you first identify the job: possession, contraction, or plural formation. Possession and contractions commonly need an apostrophe, while ordinary plural nouns do not. House styles can differ on narrow edge cases.
An apostrophe and a prime mark are different Unicode characters with different functions. Apostrophes belong to language punctuation, while the prime symbol is used in technical notation such as feet, minutes, or mathematical derivatives depending on context.
Apostrophes can mark possession, contractions, or omitted initial letters, and those uses should not be confused with quotation marks. Chicago also has specific title-case examples for initial-apostrophe contractions and for the compound form “O’Clock.”
Punctuation depends on whether the appositive is necessary to identify the referent in context. The phrase can follow or precede the noun it explains, and commas signal supplemental rather than identifying information.
Approx./approx. are documented shortened forms of approximate/approximately. This is a style-and-abbreviation question rather than a universal punctuation law.
Academic-paper titles do not have one context-free quotation-mark rule. Purdue OWL says the title on your own MLA paper is centered without quotation marks or italics, while MLA Style Center guidance puts an unpublished student paper’s title in quotation marks when it is cited.
Article titles are normally placed in quotation marks in MLA-style prose because an article is a shorter work contained within a larger publication. Required house or citation styles can differ.
ASAP/asap abbreviates “as soon as possible.” Cambridge records both uppercase and lowercase forms in Business English, so capitalization is style-dependent rather than a grammar test.
The @ symbol has legitimate literal and technical functions, but using it as casual shorthand for the word “at” can reduce clarity and accessibility. Preserve exact identifiers and official strings rather than rewriting them.
A basis point is one hundredth of a percentage point, making 100 basis points equal to 1 percentage point. The abbreviation “bps” is common in finance, but its exact typographic treatment can follow house style.
The bullet “•” is a dedicated Unicode punctuation character used in many list and separator layouts. Its visual role is typographic: list structure, indentation, spacing, and accessibility should be handled consistently by the document or interface rather than by substituting arbitrary lookalike symbols.
Bulleted lists make related, unordered information easier to scan. Strong list design uses a clear lead-in, parallel item structure, consistent capitalization, and punctuation that matches whether the items are complete sentences or fragments.
Capitalization beside punctuation is a structure-and-style decision, not a mark-by-mark automatic rule. Semicolons normally continue the same sentence; em dashes can interrupt the same sentence or precede a genuinely new one; quotations depend on whether the quoted material is a complete sentence or fragment; and parenthetical material depends on whether it is inline or a standalone sentence. Colons need an explicit house-style check because current authoritative systems differ.
The verb “cc” means to send someone a copy of an email or letter. Oxford records the past form “cc’d” and the -ing form “cc’ing”; Cambridge confirms “cc” as a verb. Other edited forms occur, but “cc’d” is a strong dictionary-supported choice when you need the inflected abbreviation.
Cf. is a comparison signal in formal writing. Its core function should be distinguished from a generic “see” reference, and its punctuation should follow the applicable editorial style.
A colon can introduce a list when the lead-in strongly points forward to the items, but it is not automatic before every list. Microsoft’s style guidance varies the punctuation according to whether the introduction is a complete sentence, a fragment, or a heading.
Use a colon when the first part establishes a clear expectation and the following material fulfills it. Colons can introduce lists, examples, explanations, questions, or quotations. Capitalization after a colon and some list conventions vary by style.
Initial yes and no often function as introductory response words rather than as grammatical parts of the clause that follows, so edited prose normally separates them with a comma. Chicago also notes that equivalent introductory phrases can take the same treatment. The rule is functional: yes/no inside another construction does not automatically receive a comma.
A comma before an “including” phrase is not automatic; punctuation can change whether the information is read as supplementary or restrictive. Chicago’s guidance makes the distinction depend on meaning, so avoid a mechanical rule that always inserts or always removes the comma.
Comma use in dates is tied to the date order and house style. Microsoft’s month–day–year convention includes a comma before the year, while Australian Government day–month–year style omits a comma. Apply the punctuation that belongs to the chosen date format.
A comma splice is a sentence-boundary error in which a comma alone joins independent clauses. The repair depends on the relationship between the clauses: use a period, a semicolon, or a comma plus an appropriate coordinating conjunction.
When an address is embedded in a sentence, commas commonly set off the city and state or comparable geographical units, while the street number and street name stay together without an internal comma. Postal-address formatting follows separate conventions.
Introductory commas mark the boundary between opening material and the main clause. Purdue OWL directly supports the comma after an introductory dependent clause; broader introductory phrases should be handled by their actual structure and clarity needs rather than by an indiscriminate “comma after every opener” rule.
Do not apply one “comma with every dependent clause” rule. First identify which clause is dependent and where it appears; then consider whether the following clause is essential to the meaning or is supplementary. Clause order and function drive the punctuation decision.
Numeric comma use is a formatting convention, not a universal property of numbers. General prose often groups digits to the left of the decimal point, while years and other identifiers commonly stay ungrouped. Locale and publisher rules can change the separator.
Quotation punctuation has two decisions: what punctuation the sentence structure requires and where that mark is placed under the governing convention. Introductory or following attribution commonly creates comma patterns, but not every quotation takes a comma.
Find the clause boundaries first. If both sides contain an independent clause, coordination can create a compound sentence; if one clause depends on the other, the result is complex instead. Punctuation depends on how the independent clauses are joined.
Conjunctive adverbs are not coordinating conjunctions. They can connect clauses, link separate sentences, or comment within a clause, so punctuation depends on where and how they are used rather than on the word alone.
Contraction choice combines grammar, spelling, and register. Writers should form standard contractions correctly, interpret ambiguous endings such as ’s and ’d from context, and follow the expected level of formality rather than applying a universal ban.
Use meaning first, then practical diagnostics such as inserting and or reversing the adjectives. If both alternatives remain natural and the adjectives have equal status, they are likely coordinate; if the adjectives build a hierarchy or unit, they are more likely cumulative.
Currency symbols and codes do not have one universal placement rule across all currencies and locales. Microsoft house style puts a currency code before a specific amount with no space, and permits a familiar symbol when the currency is clear.
Date ranges can be written with words or with an en dash depending on context and house style. Australian Government guidance prefers worded spans in ordinary prose and reserves en dashes for specified date/year contexts, while dash spacing remains a separate formatting decision.
Punctuation after “Dear Name” depends on correspondence style. Government of Canada guidance uses a colon for standard North American business letters and a comma for more personal or social business correspondence.
The degree sign participates in different conventions depending on what is being measured. The BIPM SI Brochure places a space between a Celsius temperature value and °C, while plane-angle degree symbols follow the number without a space. The Unicode character identity is U+00B0 DEGREE SIGN.
Dept/dept. abbreviates department. Cambridge shows periodless and period-ending forms across its dictionary varieties, so the useful rule is to follow the relevant regional or organizational style consistently.
Dollar amounts can be written with a dollar sign, a currency code, or the word “dollar,” depending on context and the need to disambiguate currencies. Microsoft style allows the symbol when the intended currency is clear and otherwise uses the code before the amount.
The dollar sign and USD are not interchangeable labels in every context. `$` is a familiar currency symbol, while USD is the ISO 4217 code that explicitly identifies the US dollar when international or technical clarity matters.
The double-prime character and a quotation mark are distinct. Quotation marks delimit quoted language or serve other editorial roles; double prime belongs to technical notation such as inches, seconds of arc, or derivative notation when the relevant convention defines it.
Double quotation marks commonly mark quoted words or speech, but detailed punctuation and nesting conventions vary by style and region. The project therefore treats Microsoft’s double-outer/single-inner pattern as a style rule, not a universal law for every publication tradition.
Dr. versus Dr is a style and regional formatting choice. Cambridge explicitly shows British Dr and American Dr., and Chicago uses the period-ending form, so the document’s required style should control.
E.g. punctuation is a style-mechanics topic distinct from choosing between e.g. and i.e. This guide documents MLA treatment without turning a style-specific convention into a universal grammar rule.
E.g. and i.e. serve different functions: one introduces examples and the other identifies or restates what is meant. The guide keeps that semantic choice separate from punctuation-specific guidance.
Ellipsis spacing is not one universal English rule. Microsoft’s house style uses an ellipsis to mark omitted quoted text, specifies spaces around an in-sentence omission, and permits three periods without spaces between them when the ellipsis character is unavailable.
The em dash is a flexible punctuation mark for strong breaks and parenthetical material. Microsoft style uses it for abrupt breaks, clarification, amplification, and parenthetical remarks, but warns against overuse and specifies no surrounding spaces for its house style.
Spacing around an em dash is a house-style choice, not a property of the Unicode character itself. Microsoft style uses no spaces around em dashes, while other publishers may follow different conventions.
A title appearing inside another title has its own identity and may need quotation marks, italics, or language-specific treatment. Flattening the inner title into the outer title’s token stream can destroy both formatting and capitalization.
Use the en dash only where the governing style calls for it. Some styles use an unspaced en dash for numeric spans and relationships; others prefer words such as “from … to” in general prose for accessibility, and dash treatment for parenthetical material varies by style.
Spacing around an en dash depends on the governing style and the job the dash is doing. Microsoft normally uses unspaced en dashes for numeric/date ranges but allows spaces in specified UI date/time ranges; the Australian Government Style Manual likewise varies spacing according to the elements in a span.
An en dash and an em dash are different marks. In Microsoft’s style guidance, the en dash is used for ranges and selected compounds, while the em dash marks a stronger sentence break or parenthetical interruption.
An essential clause supplies information needed to identify or restrict the noun it modifies. In relative-clause terminology it is commonly called restrictive or defining, and it is normally not separated from the main clause with commas.
Est. is a documented abbreviation with more than one common expansion. The page distinguishes meaning, capitalization, and punctuation rather than presenting one context-free rule.
Et al. means “and others” and is common in formal citation contexts. This guide treats the period as part of the abbreviation while keeping comma placement scoped to the relevant citation or editorial style.
Etc. abbreviates et cetera and normally includes a period. When it is the final item in a declarative sentence, one period is enough; formal publications may also limit when the abbreviation is appropriate.
An exclamation mark/exclamation point marks heightened emphasis or exclamation, but professional style guidance commonly recommends using it sparingly. House styles differ, and quoted material keeps its own required punctuation; restraint is a style principle, not a ban on every exclamation mark.
Separate sentence function from punctuation. A declarative or imperative can end with an exclamation mark, while a grammatical exclamative often uses “what” or “how” constructions. In formal prose, use exclamation sparingly unless the genre calls for it.
Unicode assigns FIGURE DASH and EN DASH separate code points. The en dash has established editorial uses such as ranges in many style systems; the figure dash is a distinct character associated with figure-width or numerical typography and should not be substituted automatically for the en dash.
The strings ft, in, and oz function as compact unit symbols in NIST conversion material. They are useful after numbers in technical or tabular measurement notation, while the spelled-out unit names remain clearer in prose when space is not constrained. Punctuation and symbol treatment should follow the governing technical or house style.
Fused sentences are one form of run-on construction. They differ from comma splices: a fused sentence has no suitable boundary punctuation between the independent clauses, while a comma splice uses a comma alone.
FYI/fyi abbreviates “for your information,” especially in messages and email. Cambridge records uppercase and lowercase forms, so capitalization should follow context or house style rather than a universal rule.
When a person’s name is used as direct address after “Good morning,” set the name off with a comma: “Good morning, Maya.” The same direct-address principle applies when the name appears elsewhere in a sentence, although email greeting punctuation can also be shaped by the surrounding salutation format.
For scientific and technical measurement, the recognized unit symbol for hour is “h,” and unit symbols are not pluralized. Forms such as “hr” or “hrs” may appear in general prose or house styles, but they should not be presented as the SI-style unit symbol.
When an address is presented as part of running prose on one line, commas can separate the address elements for readability. That prose convention is different from an official postal address block, where postal standards may call for line structure and minimal punctuation for machine processing.
For U.S. mailing addresses, USPS Publication 28 specifies recognized secondary-unit designators such as APT and gives placement rules for apartment information. This guidance is explicitly U.S.-postal. International address formats can follow different national standards and should not be forced into USPS form.
English euro formatting depends on whether you use the currency name, the € symbol, or the EUR code. Government of Canada guidance uses lowercase “euro,” ordinarily plural “euros,” € immediately before the amount, and a space after EUR before the number, while noting specialized legal-document exceptions.
Hyphenation improves clarity when words form a unit before a noun or when joining them avoids ambiguity, but compounds often become open or closed over time. A hyphen is not interchangeable with an en dash or em dash.
A hyphen and a minus sign can look similar in some fonts, but they do different jobs. Microsoft distinguishes the hyphen used in compounds from the minus sign used for subtraction and negative numbers, so professional text should not treat the symbols as interchangeable.
Hyphens and dashes are not interchangeable in title-case logic. Chicago 18 changed the treatment of later elements in prefix-based hyphenated compounds, while an em dash can act like a parenthetical or subtitle-like boundary depending on structure.
I.e. punctuation is a style-mechanics question rather than a different meaning of the abbreviation. This guide documents MLA treatment while keeping the recommendation scoped to that style system.
An interrogative sentence is a sentence form used to ask a question or seek information. It should be distinguished from embedded questions and from requests phrased as imperatives or declaratives.
ISO 8601 provides an unambiguous calendar-date representation in year-month-day order, commonly shown as YYYY-MM-DD. This is especially useful for data interchange and international technical work, but a human-facing publication can still require a different local or house style.
List-item punctuation depends on the grammatical form of the items and the governing house style. Microsoft omits terminal punctuation from many fragments and short labels, but uses periods for complete sentences and some sentence-completing items.
“M” and “MM” are both used for millions in different financial and business conventions, but Chicago explicitly notes that neither convention is universal. “MM” can represent a thousand thousands in one finance convention, while many modern contexts use “M” for million. Define the convention when ambiguity matters.
Mr. versus Mr is primarily a regional and house-style formatting choice. Chicago uses the period-ending form, while Cambridge’s British entry presents Mr without a period, so consistency with the chosen style matters.
The multiplication sign “×” and the letter “x” can resemble each other but are not the same character. Unicode identifies × separately, and the SI Brochure uses the multiplication sign in scientific notation. Dimensions, variables, and plain-text contexts can follow different conventions, so context still controls.
“Namely” and “i.e.” both introduce more exact information, but they are not identical formatting choices. Namely is a word meaning roughly “to be specific,” while i.e. is the abbreviation of Latin id est, “that is,” used for clarification or restatement.
A colon can create a title/subtitle boundary, but nested punctuation still needs structural handling. Chicago capitalizes the first word after a title/subtitle colon; repeated or nested colons should therefore be parsed as boundaries rather than treated as ordinary word punctuation.
“No.” is a conventional abbreviation for “number” in Chicago-style contexts. The guide keeps it distinct from the number sign and from shortened forms such as num. or n., which may carry other meanings.
A nonbreaking hyphen and a regular hyphen are different characters. Unicode line-breaking rules treat U+2011 NON-BREAKING HYPHEN as a hyphen-like character that prevents a line break at that point, so it is useful only when the no-break behavior is intentionally required.
A nonbreaking space behaves like a space while preventing a line break at that position. In professional documents it can keep selected words or short units together, but it should not be used as a manual layout or indentation tool.
A nonessential clause adds information without restricting which person or thing the sentence identifies. In standard edited English, it is set off with commas when embedded in a sentence; an essential/restrictive clause is not treated the same way.
An en dash can compactly show a span such as 2016–2020 or a page range. In prose, however, a construction introduced by “from” should normally be completed with “to” or “through” rather than mixed with an en dash.
P.M. and PM are formatting alternatives, not different meanings. Chicago documents several legitimate forms and prefers lowercase “p.m.” in its own style, while other publications may choose uppercase or periodless forms.
Page abbreviations are style-dependent. MLA documents p. and pp. in relevant works-cited contexts, while Chicago often omits them but allows them for clarity or consistent house style; “pg.” should not be substituted automatically.
Parentheses can appear in headings and titles without creating a new capitalization system. Chicago says the parentheses themselves are normally ignored when title case is applied, so words inside still follow their grammatical title-case rules.
Parentheses signal that enclosed material is secondary to the main sentence. Keep them sparse, distinguish them from square brackets, and place surrounding punctuation according to whether the parenthetical is part of another sentence or is a complete standalone sentence.
Parentheses and square brackets should not be swapped merely for appearance. Professional styles commonly use parentheses for parenthetical material and square brackets for functions such as editorial insertions or nested parenthetical material; exact conventions should follow the governing style guide.
Whether to use “%” or spell out “percent/per cent” is a house-style decision rather than one universal English rule. Microsoft style uses % with numerals in ordinary cases, while another public-sector style guide prefers words in most prose and reserves % for tables, demonstrating legitimate editorial variation.
Percentage points and percent describe different calculations. The difference between 10% and 9% is 1 percentage point, while a 1% relative decrease from 10% would produce 9.9%, not 9%.
The sentence-ending rule is simple; the exceptions are mostly formatting and house style. Use a period to close ordinary declarative sentences, then check the relevant style guide for abbreviations, lists, headings, captions, interface text, web addresses, and other special contexts.
Degree abbreviations are style-dependent. Current MLA Style Center material uses “PhD” without periods in editorial prose and author biographies. Other institutions may prescribe “Ph.D.”, so writers should follow the style guide governing the document rather than treating punctuation as a grammar error.
The plus-minus sign is a dedicated character rather than a decorative pairing of plus and minus marks. Unicode identifies it as U+00B1 PLUS-MINUS SIGN. Its exact mathematical or technical interpretation depends on the field, so professional copy should preserve the notation required by the source or governing standard.
Possessive apostrophes become difficult when several names share or separately own something, or when a singular name already ends in s. The key is to decide the ownership relationship first, then follow the governing style for s-ending names. This guide keeps those decisions together because they are variations of the same possessive-structure problem.
NIST measurement tables use “lb” for pound. In measurement notation, the symbol itself does not become “lbs” just because the quantity is plural. General prose and informal packaging sometimes use “lbs,” but technical writing should follow the applicable measurement or house standard.
`£` is the familiar pound sign used in currency formatting, while GBP is the ISO 4217 code for pound sterling. The code is especially useful in international, financial, or machine-readable contexts where explicit identification matters.
Chicago 18 changed how later elements in prefix-based hyphenated compounds are capitalized: each component now follows the general title-case rule. Safe automatic treatment still requires recognizing that the hyphen truly joins a reviewed prefix compound.
Some names and work titles intentionally use distinctive casing or punctuation that a generic title-case algorithm should not normalize away. A safe converter needs a reviewed preservation layer and must distinguish a known protected form from arbitrary mixed case.
Colons, semicolons, and dashes can create capitalization boundaries in titles, while quotation marks and parentheses may introduce embedded structures that require additional analysis. This guide separates punctuation-boundary detection from word-class decisions so it can explain when a new segment begins and when it must leave the result unresolved.
Decide first whether the sentence is actually asking a direct question, then decide what exact text the question mark applies to. Do not add a question mark merely because the sentence contains a question word such as who, why, or whether.
Quotation marks do not by themselves decide which words are capitalized. A converter must distinguish harmless outer quotation punctuation from an embedded title or quoted structure that may need its own capitalization and formatting rules.
This guide covers representation of ratios in writing, not how to calculate them. Formatting choices should make the relationship easy to read and should yield to domain-specific or publisher conventions when those differ.
Ref/ref. is a documented business abbreviation for reference. Cambridge also records an unrelated informal “ref” for referee, so context matters.
First confirm that the problem involves two independent clauses. Then choose a repair that matches the logical relationship: make separate sentences, use a semicolon, use a comma plus coordinating conjunction, or restructure one clause as dependent.
A semicolon is stronger than a comma but does not by itself turn a dependent clause into an independent one. Check that the major units on both sides can stand as complete clauses when using the core clause-joining pattern, and follow house style for lists.
The “serial comma” and “Oxford comma” name the same list-comma position: immediately before the final conjunction in a series. Microsoft requires it in its house style, but professional writing should follow the relevant publication or organizational style rather than treating the preference as universal grammar.
Single quotation marks can be primary quotation marks in some style systems and nested quotation marks in others. Australian Government style, for example, uses single marks for most direct speech and double marks inside them.
A slash can join alternatives, words, phrases, or other structures, so the slash itself is not enough information to decide capitalization around it. This guide therefore treats unresolved slash constructions as structural cases rather than splitting and guessing.
A slash has legitimate uses in established expressions, paths, and URLs, but style guides caution against using it as a vague substitute for “or” in ordinary prose. A slash can mean different relationships in different contexts, so do not assume readers will infer whether it means “or,” “and,” “per,” or a structural separator.
The slash `/` and backslash `\` are different characters with different technical uses. Unicode identifies them as SOLIDUS and REVERSE SOLIDUS, and Microsoft writing guidance distinguishes slash use in web addresses and general constructions from backslash use in server or folder paths.
“Smart” or curly quotation marks use directional opening and closing characters, while straight quotation marks use non-directional forms. Unicode distinguishes the directional characters, and Microsoft’s own Segoe guidance uses straight quotation marks, demonstrating that the right choice depends on environment and style rather than a universal preference.
The first word of a subtitle is treated as a new capitalization boundary in major title-case systems. APA explicitly capitalizes the first word of a subtitle, Chicago capitalizes the word after a title/subtitle colon, and MLA has published the same treatment in its Style Center guidance.
Table-cell punctuation should reflect the grammar and the document’s style. Microsoft uses end punctuation for cells containing complete sentences or mixed sentence/fragment content, while simple fragments can remain unpunctuated.
Quotation-terminal punctuation requires two decisions: which text owns the punctuation, and which regional or house convention governs periods and commas. Question and exclamation marks are primarily logical; period placement can differ across style systems. Avoid doubled terminal punctuation when one mark already closes the sentence.
Time-range formatting depends on context. Microsoft recommends “to” in running text and an unspaced en dash in many schedules, while Australian Government guidance varies en-dash spacing according to AM/PM and words such as noon or midnight.
Legal writing is style-governed. “Versus” means “against” in a case name, but the shortened form depends on the citation system, jurisdiction, publication, or court rule. Do not transfer a general-prose “vs.” preference into a legal citation without checking the controlling style.
The vertical bar “|” and slash “/” are separate Unicode characters. Their technical meanings depend on the syntax in use, while editorial slash usage follows a different set of writing conventions.
“Viz.” is a written abbreviation used to introduce a more exact specification of what was just stated. Cambridge marks the form old-fashioned and gives “namely” as a modern synonym.
There is no single universal rule for “versus” and “vs.” across all writing. Microsoft’s style guide spells out “versus” in running text and uses lowercase “vs.” in headings. Other disciplines may use different conventions, so the safest choice is to follow the governing style guide and stay consistent.
Use “1990s,” without an apostrophe, for the plural name of the decade. The Australian Government Style Manual explicitly rejects “1990’s” for that plural; an apostrophe is appropriate only when it marks omitted digits in a shortened form such as “the ’90s.”
The yen sign `¥` is a familiar currency symbol but is not an unambiguous international identifier by itself. JPY is the ISO 4217 code for the Japanese yen and is clearer when the specific currency must be explicit.
In a formal business letter, “Re:” can introduce an optional reference line identifying the file, invoice, matter, or subject. Government of Canada guidance gives the form “Re: Invoice 62395” and notes that a Subject line can be used instead. Email reply prefixes are a separate system convention.
