"Let's eat, Grandma."
See? Commas really do save lives.
All kidding aside, let's not knock the ubiquitous comma. It's the most common punctuation mark, which pretty much guarantees it's the most misused one. Writers have a lot of leeway with commas, but there are some rather hard and fast rules.
A little history might help. Back in the day, when most people had to have books read to them, commas provided a visual cue to the reader: Take a brief pause here! Here is a break in the rythmn! In our age of widespread literacy, commas serve much the same purpose. They set off sentence parts, indicate short pauses, and help avoid confusion. If you think of them that way, the rules will make more sense. Most of the time. There are exceptions, I'm afraid, but we'll get into those later.
Everyone wants to know about the serial comma, so I'll talk about that first. When the text includes a series of words or phrases, the preferred usage demands a comma between them. When the final words/phrases are joined by a conjunction, put the comma before the conjunction. The flag is red, white, and blue. I ate breakfast, went for a walk, and collected the mail. My brother likes steak, I prefer chicken, and my sister is vegan.
These sentences would not be incorrect without the final commas, but take a second look at the final sentence. Suppose it was in a book, and the line broke after the word "sister."
My brother likes steak, I prefer chicken and my sister
is vegan.Without the comma after chicken, a slight chance exists that someone could think I prefer (to eat) chicken and my sister. Eww.
If all the elements of a sentence are short and joined by conjunctions, you don't need any commas. Sally ordered eggs and bacon and toast. If they're long or complicated, use commas to indicate pauses. If you think about reading the text aloud, you'll see where you need to insert commas.
When items in a series include internal punc of their own, separate them with semi-colons. I'd like to introduce my mother, Ginny; my father, Walt; my brother, Tom; and my sister, Peg.
(I might as well point out here that, yes, you do need a comma after father, mother, brother, sister unless you have more than one of any of them. My brother Tom [no comma] indicates "not my brother Dave." This is one of those nitpicky rules you just have to learn.)
Note that some writers and publishers don't like the serial comma. Be guided by your publisher's wishes and, above all, by clarity. If a comma would make your writing clearer, by all means use it.
Okay, that's enough for one lesson. More on commas next week.