Punctuation and Mechanics: Commas, Semicolons, Colons, Apostrophes
Commas, semicolons, colons
Use commas to set off introductory elements, to separate items in a series, to mark nonrestrictive (extra, removable) information, and before a coordinating conjunction joining two complete clauses. Do not use a comma between a subject and its verb, or to join two clauses by itself.
A semicolon joins two closely related complete clauses or separates series items that already contain commas. A colon must follow a complete clause and introduces a list, explanation, or quotation.
Restrictive versus nonrestrictive information is a favorite trap. The report that was due Friday is late is restrictive: it tells which report, so no commas. The report, which was due Friday, is late is nonrestrictive: extra information, so commas are required.
Apostrophes and capitals
Apostrophes show possession or contraction. Singular: the manager's plan. Plural ending in -s: the managers' plan. Plural not ending in -s: the children's plan. Its is possessive; it's means it is. Do not use apostrophes for simple plurals (three memo's is wrong).
Capitalize proper nouns, the first word of a sentence, and titles before names (President Diaz) but not general job titles (the president of the company). Capitalize directions only when they name a region (the South); do not capitalize seasons.
Traps
- Comma splices repaired with a semicolon, not an extra comma.
- A colon placed after such as or including, which is redundant.
- A missing comma after a long introductory phrase.
- Hyphenating compound modifiers before a noun (a well-known author) but not after (the author is well known).
Worked example
We visited Denver Colorado, and Santa Fe New Mexico. City–state pairs need commas: We visited Denver, Colorado, and Santa Fe, New Mexico.
Rule of thumb: if the marked-off words could be lifted out without changing which thing you mean, they need commas.