When do you 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

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

(adapted from the sixth edition of the APA Publication Manual, © 2010)