When can a comma be used before the word 'and'?

Question

Possible Duplicate:
Should I use a comma before “and” or “or”?

Is it appropriate and/or advisable to use a comma before 'and' in some situations?

Answer 1

Commas before and are useful in cases where there are two independent clauses that require linkage; for example:

A cat can climb trees, and it can climb curtains.

[A cat can climb trees] + [A cat can climb curtains].

Answer 2

Yes, it can be appropriate and advisable. Some examples from Carey's "Mind the Stop" are helpful (p49 in the 1980 reprint of the Penguin edition):

• It is impossible, and indeed undesirable, to lay down hard-and-fast rules on this subject.
• I stayed with him most of the afternoon and the next day, on going to see him again, found him a little better.

The first shows a case where the comma before and is very necessary, and the second shows a case where the absence of the comma leads you astray on first reading the sentence.