What are the rules for "and" - grammar issue

[metagirl]

When writing "and"  in a sentence, can you always do away with a comma before? - or are there cases when you should put a comma before?

Egs:
I had a cow, chicken, and two fishes.
Monkeys are used on photo shoots and commercials, and in the theatre.

[Clanad]

The comma between the word "and" and the preceding word�is often called the serial comma or the Oxford comma. Its use appears to be optional. In the past, it was considered improper to omit the final comma in a series, but modern writers believe that the conjunction (and, but, or) does the same thing as a comma: it marks the place between two items in the set. These writers have argued that a sentence is more economical without an unneeded comma. As a result, you now have the option to choose whether to include the final comma.

Nevertheless, many people still follow the old rule and expect to see the final comma.

[Emily Ball]

No comma needed after'and'. e.g.-

I had a cow, a chicken and two fishes.

Monkeys are used on photo shoots, commercials and in the theatre.

[purpose]

Due to the fact that it DOES appear to be optional in many instances, I find it useful to read the sentence aloud, as this (for me) is a good measure of whether an Oxford comma SOUNDS right, rather than looks right, because I think it looks sooooooooooooo wrong!

[metagirl]

thanks for that. I tend to do what purpose recommends and only use the comma when I actually pause in speech. do you think there's a difference in comma usage between the US and UK?

in the US, it seems that commas are used more frequently (at least that was the case when I was in school) than in the UK, so I had to re-learn the rules, basically. and as you can tell by question, I'm still sometimes puzzled!

[Quizmonkey]

Emily Ball's second sentence is ungrammatical - because of the use of two different preopostions. The followng are correct:

...are used in photos, commercials and the theatre. (because all three list items are governed by the same preposition).

but:

...on photo shoots and television, and in commercials.

There's a huge thread on the Oxford comma a few pages back.