Fill-in vs. Fill In
“Fill-in” is a hyphenated noun for a temporary substitute and can also be an attributive adjective. “Fill in” is the two-word phrasal verb used for completing blanks, substituting for someone, or supplying missing information.
Use “fill-in” for a temporary substitute person or, in mainly US use, as an attributive adjective. Use “fill in” as the verb for completing information, substituting temporarily, or bringing someone up to date.
Key details
Choose the right form
| If you mean / need | Use | Why |
|---|---|---|
| a temporary substitute person | fill-in | Cambridge records the noun with a hyphen. |
| temporary substitute before a noun | fill-in | Cambridge records an attributive adjective, mainly US. |
| complete information or substitute temporarily | fill in | The phrasal verb is written as two words. |
Further guidance
Core difference
Use the documented noun or modifier form when naming a thing, result, person, or concept; use the spaced form when the words function as a verb.
Fast Test
If the expression can take ordinary verb tense or inflection in the sentence, the spaced verb form is usually the relevant choice.
Variant Boundary
Cambridge distinguishes the hyphenated noun/adjective “fill-in” from the phrasal verb “fill in,” which has several senses.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- FILL SOMETHING IN — English meaning (opens in a new tab)Cambridge Dictionary · Hyphenated noun/adjective “fill-in” and phrasal verb “fill in”