The Comma Demystified

Essential to writing is the humble comma. Thought by many the creation of a sadist, the dot-curl combo can be the key to clarity. My comprehensive school education certainly skirted the issue. So, as a mature student, I looked into it myself. I found a few simple rules in a simple book and, in a few hundred words, here is most of what you need to know about the comma, but were afraid to ask.

Commas are not to be used to give readers a chance to breathe: they have had plenty of practice at that. No, there are four, and only four, distinct uses of the comma: listing, joining, gapping and bracketing.

The listing comma separates a lists and substitutes for the word “and” and sometimes the word “or”.
“The three musketeers were Porthos and Aramis and Athos” becomes
“The three musketeers were Porthos, Aramis and Athos”.

There are two extra rules for the listing comma. Listing comma rule one: use it only to separate a list of three sentences or more.

“I speak English, she speaks Italian and he speaks Russian” is correct, but
“I speak English, she speaks Italian” is wrong.
Think of this rule as that a list of two sentences is just a join between the sentences, not a list.

Listing comma rule two:  if, in your list, one of the items has an “and” put in a comma before this “and” for clarity.
“He liked listening to the Beatles, Status Quo, and Gilbert and Sullivan” is good, but
“He liked listening to the Beatles, Status Quo and Gilbert and Sullivan” is confusing.

The general test for a joining comma is whether you could substitute “and” or “or” and the sentence would still make sense. The exception is when you might need a comma before the “and” or “or” for clarity when an item in your list has an “and” or “or” itself.

The joining comma joins up two complete sentences, but must be followed by one of these joining words: “and”, “or”, “but”, “while” and “yet”.

“His shirt is blue, but her shirt is red” is correct, but
“His shirt is blue, her shirt is red” is wrong.

Note that the joining words “and”, “or”, “but”, “while” and “yet” follow this rule. However, other joining words should by proceeded by a semi-colon if the sentences are closely related or should start a new sentence. Examples of joining words that do not receive the comma treatment are: “however”, “therefore”, “hence”, “consequently” and “nevertheless”.

“His shirt is blue, however her shirt is red” is wrong.

The gapping comma shows that some words have been left out when those missing words would just repeat words used earlier in the same sentence.

For example:
“Churchill was famous for his cigars, Hitler, for his moustache, and John Wayne, for his walk”.
Here the words “was famous” have been substituted by a gapping comma. Sometimes this is not necessary if the meaning is clear anyway.

The most common, and difficult, use of the comma is the bracketing comma is a pair of commas used to isolate an interruption to the main thrust of the sentence. If the interruption is at the beginning or end of the sentence then, of course, only one bracketing comma is needed.

For example
“These findings, we would suggest, cast doubt upon the hypothesis” or
“These findings cast doubt upon the hypothesis, we would suggest”

The first test here is whether removing the interruption would leave a sentence that makes sense.
“She groped for her cigarettes and, finding them, hastily lit one” is good, but
“She groped for her cigarettes, and finding them, hastily lit one” is WRONG because removing the interruption leaves a sentence that makes no sense.

The second test is whether the interruption is just that, an interruption, not an essential part of the sentence. So:
“Note that in this example, the sentence remains clear” is wrong because the words “Note that”, as an instruction, are vital to the central thrust of the sentence.
“Note that, in this example, the sentence remains clear” is correct.

These concepts were taken from the Penguin Guide to Punctuation by R. L. Trask. I suggest you get one. If only they had taught me this at school!