Comma before 'and'

When is it appropriate to put a comma before the word 'and'? I always thought this was a definite no-no, but I am trying to settle a debate.

Can anyone advise? Many thanks in advance.

[MrPedantic]

Hello Cowgirl

It's always correct to put a comma before "and" where ambiguity would otherwise ensue:

1. I want to thank my parents, Sam and Dave.

Unless you are indeed the result of some curious monosexual coupling, change to:

1a. I want to thank my parents, Sam, and Dave.

Elsewhere, there are two schools of thought. School A says that where the last term of a list is introduced by "and", it is redundant to put a comma before that "and". Thus School A prefers:

2. I bought some apples, pears and oranges.

School B on the other hand thinks that the comma before "and" denotes a pause, and is therefore not redundant. Thus School B prefers:

2a. I bought some apples, pears, and oranges.

(For some reason, lists of fruits are obligatory in this particular debate.)

In British English, School A is predominant, especially in newspapers and magazines. However, some academic publishers (notably Oxford University Press; whence this comma's name, the "Oxford comma") belong to School B.

MrP