In the last post, I said that commas are used to indicate the separate items in a list. That function is just an extension of the comma’s basic purpose, which is to show where a sentence’s joints are: where introductory, interrupting, or concluding phrases and clauses begin and end, which are the units in a list or the parts of an address, a person’s title, where the non-essential clauses are, and so on.
Some people find it useful to think of commas as showing where you should pause when reading a sentence, but that’s an oversimplification. If you stop with that rule of thumb, I think you run the risk of leaving out commas that should be added, and adding commas that you shouldn’t.
With that in mind, here’s another use for commas: Insert a comma when you have two complete sentences joined with a conjunction, such as and, but, yet, because, and so on. For example:
Almost everybody knows what a comma is, but few can explain how to use it.
That sentence really contains two complete sentences: "Almost everybody knows what a comma is," and "Few can explain how to use it." This causes trouble for some people when they omit the conjunction; what you have then is lovingly called a comma splice, and it looks like this:
Almost everybody knows what a comma is, few can explain how to use it.
Please don’t do this. If you want to leave out the conjunction, then use a semicolon instead of a comma. Yes, it is true that you can find examples of comma splices (also known as splice commas) in the writings of such literary luminaries as Samuel Beckett, E.M. Forster, Somerset Maugham, and John Updike, just to name a few. But you’re not them, are you? e e cummings wreaked havoc on rules of spelling, capitalization, and punctuation, but if you’re not him, I don’t recommend that you follow his lead either. Lynn Truss (I mentioned her and her book Eats, Shoots, and Leaves in my previous post) explains it this way:
Now, so many highly respected writers adopt the splice comma that a rather unfair rule emerges on this one: only do it if you’re famous.
Unfair, maybe, but it works for me.