Everything You EVER Wanted to Know About Commas, But Were Afraid to Ask

This is the most straight-forward explanation of commas you'll ever get. I originally wrote it for my students at Pulaski Technical College and have republished it here for my writing students at Lingua Espresso.

In the beginning, was the simple sentence with one subject and one verb, as in "He played basketball."

Later, came the compound sentence. This means that two complete sentences, each which can stand alone, are joined together with appropriate punctuation. Incomplete clauses cannot stand alone and are called fragments (in other words, "bits" of sentences). For example,

1. After it rained.
2. On top of the mountain.

Both of these are fragments. Sentence One is a fragment because it is a subordinate clause: it leaves you hanging. After it rained, what then? Sentence Two is a fragment because it is simply two prepositional phrases, and we know subjects and verbs aren't found in prepositional phrases. So to have a complete structure, you need to add a complete sentence with a subject and verb:

1. After it rained, we were soaking wet.
2. We were soaking wet after it rained.
3. On top of the mountain, we had a picnic.
4. We had a picnic on top of the mountain.

Around 25% of all commas are used to join introductory material such as subordinate clauses ("after it rained") and prepositional phrases ("on top of the mountain") to the beginning of a complete sentence. Notice there is no comma if that material falls at the end. Such sentences are called complex because they have one stand-alone and one subordinate clause/sentence.

Now, 50% of all commas are simply used to compound, to join two complete sentences together. If you don't use the proper punctuation to join two complete sentences together, you get a run-on sentence or its variant, the comma splice. For example:

1. I love to play basketball I really love to play football. (typical run-on)
2. I love to play basketball and I really love to play football. (typical run-on)
3. I love to play basketball, I really love to play football. (typical comma splice)

So what to do? Well, the easiest thing to do is to break them with a period:

• I love to play basketball. I really love to play football.

However, this can get quite choppy if you do it all the time. The second easiest thing to do is to divide them with a comma and a coordinating conjunction ("and", "but", "or", "for", "so", "nor", and "yet"):

• I love to play basketball, but I really love to play football.

Remember you can't have just the coordinating conjunction or just the comma: you must have both.

Other methods:

You can add a word to make one sentence a subordinate clause as in:

• love to play basketball while I really love to play football.

Or:

1. I love to play basketball; I really love to play football.
2. I love to play basketball: I really love to play football.

Both the semicolon and the colon are strong enough to separate two sentences on their own. However the thoughts must be closely related. You can't say:

• I love to play basketball; soup is good food.

Now, the most common marks of punctuation between two sentences will be the comma and coordinating conjunction or the semicolon. The colon is even stronger than the semicolon, and the dash is strongest of all and always used for rare dramatic emphasis, as in:

• I only fear one thing — that thing is death.

So, in sum, 25% of all commas are used with introductory phrases and clauses, while 50% are used with a coordinating conjunction between two complete sentences. So where is the other 25%? Spread out in diddly things like this:

1. We bought bread, butter, ham, and mustard. (commas with items in a series)
2. She was a smart, pretty, innocent person. (commas with multiple adjectives before a noun; if you can put the word "and" in and it make sense, you should put a comma.)
3. Hopefully, you'll learn your lesson (commas with introductory words).
4. I know, for example, that you are a liar (unrestrictive words or phrases dumped into a sentence; in other words, you can take out "for example" and the sentence will still make sense.)
5. My parents want me to be a doctor; however, I want to be a dentist (words like "however" and "therefore" used to divide two sentences require a semicolon before, and a comma after.)

Now, I hope this has helped. It is not hard, but it does require careful proofreading and the understanding of subjects, verbs, and complete sentences to get this right.