Punctuation rules

On the FEATURED: WRITING TIPS & TRICKS thread, I posted a long discussion of the rules of punctuation with quotation marks.

Basically, there is the American style (where commas and periods are always inside the ending quotation mark), and the British style (where commas and periods are outside, unless the punctuation is part of the actual quote). For question marks (and other punctuation), the Brits and Yanks agree -- outside unless the question mark is part of the original quote. See the FEATURED thread for more detail.

There are two situations where a comma must be placed before "and" -- compound sentences and what is called the "serial comma."

Yesterday, I described (in another thread) compound sentences -- two independent clauses joined by a comma + conjunction. The most common conjunction is "and," but there are others as well.

Henry hit the ball, and Joey ran to third base.

Mary likes cake, but Sally prefers ice cream.

The comma is not required when a conjunction is used to create a simple compound subject or simple compound object. These are not compound sentences.

Mary and Sally are going to a party.

Joey will serve cake and ice cream at his party.

Items in a series (that is, three or more items) are separated by commas. When a conjunction joins the last two elements in a series, a comma is required (according to most style guides) before the conjunction. Again, "and" is the most common conjunction, but the rule applies to other conjunctions as well.

Joey will serve chocolate cake, apple pie, and vanilla ice cream at his party.

I should point out that newspapers (to save space) generally do not follow this rule.