When do you use a comma?

This page reflects guidance from the sixth edition of the Publication Manual.

For the most current guidelines, see the Publication Manual of the American Psychological Association (7th ed.) and our Style and Grammar Guidelines page for the seventh edition.

Use a comma

between elements (including before and and or) in a series of three or more items.

the height, width, or depth
in a study by Spencer, Girard, and Singh (2010)

to set off a nonessential or nonrestrictive clause, that is, a clause that embellishes a sentence but if removed would leave the grammatical structure and meaning of the sentence intact.

Switch A, which was on a panel, controlled the recording device.

to separate two independent clauses joined by a conjunction.

Cedar shavings covered the floor, and paper was available for shredding.

to set off the year in exact dates.

April 25, 2011, was the correct date.

but

April 2011 was the correct month.

to set off the year in parenthetical reference citations

(Harper, 2012)
(Nguyen, 2009, demonstrated....)

to separate groups of three digits in most numbers of 1,000 or more

Do not use a comma

before an essential or restrictive clause, that is, a clause that limits or defines the material it modifies. Removal of such a clause from the sentence would alter the intended meaning.

The switch that stops the recording device also controls the light.

between two parts of a compound predicate

The results contradicted Smith's hypothesis and indicated that the effect was nonsignificant.

to separate parts of measurement

8 years 2 months
3 min 40 s