The comma is quite an expressive punctuation mark and is governed by a set of fairly complicated rules or guidelines. One of the most accessible descriptions of its use can be found in Perfect Punctuation by Stephen Curtis.

The comma acts as both a linker and a separator.

Commas are used to link the items in lists of words. phrases and clauses of the same type, usually replacing and. If the comma cannot be replaced by and, it is in the wrong place.

There are two systems for using commas in lists: A, B and C and A, B, and C. The final comma in the second system is called the ‘serial comma’.

Commas also link subordinate clauses to main clauses.

Commas acting as separators generally work in pairs, except when they are placed at the beginning or end of a sentence.

They also mark off sentence adverbs.

Commas are…