Commas and

I have recently noticed a lot of people using comma + and when they shouldn't and not using it when they should. It's like somewhere the rule has gotten reversed. I'm not sure how or where this happened. Perhaps it's just that in my students' overcorrection they have completely 180'd the comma rules.

But I'm guessing it's more that people have just forgotten when to use comma + and and just hope that no one reading their work knows how to do it either. I've also noticed that people seem to try to correct the blue underlines from Microsoft Word Grammar check by inserting commas. Blue underlines do not always mean a problem with commas. Furthermore, blue lines do not even mean that something is necessarily wrong. Blue lines mean that something you have written has triggered some algorithm based on inflexible grammar rules to signal you that you have written something possibly incorrect. You have to review it yourself to be sure, though, because language is not a closed system with only a few possible constructions of correctness. There are infinite ways to write the same idea, and grammar check cannot make sure that infinite ways are all correct.

So, when to use comma + and and when not? There are two grammatical structures where a comma may appear before "and". In one of these structures the comma is an absolute requirement. In the other, it is a choice.

1) Comma + and required: A comma before "and" must appear when the comma + and works to combine two complete sentences. For example:

I am going to the store, and I am going to the Post Office.

Here we have two complete sentences. The comma + and could be replaced by a period.

I am going to the store. I am going to the Post Office.

But the writer probably joined them to emphasize to the audience that she is going to both places in one trip.

2) Comma + and optional: A comma before "and" may appear before the last item in a list. This comma, sometimes referred to as the Oxford comma, is optional. However, optional doesn't mean you can decide willy-nilly when to use it and when not to. The use of this comma, whether present or absent, must be consistent within one document, so it is best to choose your personal preference and live with that decision, forever. The optional comma looks like this:

I am going to the store for bread, milk, and eggs.

The sentence is also correct written without the comma, as long as every other list in the same document doesn't include this comma either.

3) Comma should not be present with and: No comma should appear with "and" in a compound subject or predicate. Compound means two. For example:

My sister and I are going to the store.

No comma appears before "and" because "My sister and I" are a unit as the plural subject of this sentence.

I am going to the store and the Post Office.

No comma appears before "and" because "the store and the Post Office" are a unit as the plural destination in this sentence.

I am going to the store and stopping by the Post Office.

No comma appears with "and" because "am going and stopping" form the compound verb; both use "I" for the subject.



So, the final summary: the comma + and must be used to join two complete sentences, may be used before the last item in a list, and is never used with compound subjects or predicates.