Colons

The colon is a punctuation mark that signals that what follows explains, elaborates, or enumerates what precedes. It derives from Latin colon, ultimately from Greek kolōn meaning “clause” or “part of a sentence.” The mark is used to connect independent clauses, introduce lists, explanations, exemplifications, and quotations.

In English writing, common uses include introducing a list after a complete sentence (There are three options: apples, oranges, and pears); introducing an explanation or example (She had one hobby: collecting stamps); and preceding a quotation when the quotation explains the preceding thought (He replied: “We will see what happens”). It also marks time and ratios (07:30, 3:1) and appears in titles and subtitles (Economics: A Practical Guide). It is used in formal letter salutations (Dear Sir:).

Formatting rules include no space before a colon and a space after it in standard English typography. After a colon, capitalization of the following word is optional and depends on style: if the text after the colon forms a complete sentence, capitalization is common; otherwise it is usually not capitalized. In other languages, typographic conventions may differ.

In other domains, colons serve similar functions: to separate related elements or to introduce material that belongs with what preceded. In computing and digital text, colons appear in URLs (http://...), in time and ratio notation, and in programming languages to indicate blocks or labels.

The colon is a versatile punctuation mark with multiple conventional uses, and its management varies by language and style guide.