For the ordinary verb or noun meaning “check again,” major dictionaries use the hyphenated form “double-check.” An open “double check” can arise in other structures and has a specialized chess sense, so it should not be treated as a universal substitute; the closed “doublecheck” is not the general dictionary form used here.
ReviewedEvidence2 sourcesSectionEnglish Usage
Quick answer
For the general “check again” meaning, use the dictionary form “double-check”; treat “double check” according to its sentence or specialized context rather than as the default spelling.
Key details
Confusion Familyhyphenation-and-word-boundary
Rule Familyword-boundary
Termsdouble-check, double check, doublecheck
Further guidance
Core difference
“Double-check” is the ordinary dictionary headword for checking again. The open wording can have a different structural or specialized use rather than being the default spelling of that verb.
Usage Boundary
Use the hyphenated form for the general check-again verb/noun unless a specific style or syntactic context clearly calls for something else; do not default to the closed “doublecheck”.
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.
“Anyone” is one word when it means any person. “Any one” is written as two words when the construction refers to a single member of a larger set, including the common pattern “any one of …”.
“Everyone” is one word when it means every person. “Every one” is two words when one is a separate element referring to each individual member of a set, especially in constructions such as “every one of them.”
Both “insofar as” and the open form “in so far as” are established ways to express “to the extent that.” The closed form is a dictionary headword, while references also record the open construction, so house style and consistency can determine the choice.
The standard expression is written “no one,” and Cambridge also records the hyphenated variant “no-one.” The fully closed spelling “noone” is not the accepted form in the cited grammar guidance.
Both “someday” and “some day” are established. Someday is a one-word adverb referring to an unknown future time, while the open form also exists and can be the ordinary determiner-plus-noun construction “some day.”
“Someone” is the standard one-word pronoun for an unspecified person. The two-word sequence “some one” is possible when “one” remains a separate word and the construction singles out an individual member of a group, so the forms should not be treated as freely interchangeable.
