Commas, or their absence, can change the meaning of a sentence in profound ways. This is the main reason why commas must be correct. The ability of commas to change the meaning of sentences is most apparent with the word who.
To understand how this occurs, we must first understand the difference between restrictive and non-restrictive clauses.
Restrictive Clauses vs. Non-restrictive Clauses
Restrictive clause: Information that identifies which person or thing you mean and that indicates one person or thing from similar people or things. A restrictive clause restricts a reader’s attention from a category or group to a single person or thing in the category or group. Information in a restrictive clause is essential to understand the writer’s meaning. Restrictive clauses begin with that or who.
Non-restrictive clause: Information that provides an additional description of a person or thing. The reader already knows which person or thing is being described, so this information is not essential to understand the writer’s meaning. Non-restrictive clauses begin with which and who.
Commas with Who
Notice that both restrictive and non-restrictive clauses can begin with who. This means the reader will need additional clues to understand whether the writer is using a restrictive or non-restrictive clause. The reader needs additional clues to determine whether the writer is providing information to indicate which person or is providing additional information about a person. Commas provide this clue. Let’s see how.
1. I respect soldiers who always do their duty.
2. I respect soldiers, who always do their duty.
Both examples have the clause who always do their duty. In example 1, the clause is not preceded by a comma. In example 2, it is. And this makes all the difference in meaning.
Example 1 has no comma before who. This indicates that the clause is restrictive. It is essential information to understand the meaning of the sentence. The clause cannot be separated from the main message because it is part of the main message.
One category of people is soldiers. This category may have all kinds of people in it, all known as soldiers. As a restrictive clause, who always do their duty communicates which soldiers I respect. It restricts the reader’s attention from all soldiers to particular solders. I may respect other people in the category of soldiers, but here I am discussing only soldiers who do their duty.
Example 2 has a comma before who always do their duty. With a comma, this clause is non-restrictive. It is not essential information to understand the meaning of the sentence. Instead, it provides additional information about soldiers. The comma separates the clause from the main message of the sentence, which is that I respect all soldiers.
This sentence doesn’t indicate that I respect only certain soldiers. Instead, it communicates that I respect all soldiers. The non-restrictive clause provides additional information about soldiers, stating that soldiers do their duty.
Summary of the examples:
Example 1 (restrictive, no comma): I respect certain soldiers. (Zen Comma rule V)
Example 2 (non-restrictive, comma): I respect all soldiers. (Zen Comma rule U)
Value of Commas
As we see from these two examples, the comma, or lack thereof, makes a fundamental change in the meaning. Without a clear understanding of how to use commas, a writer risks communicating the wrong idea to the reader.
Need help with commas? Get Zen Comma, an instructive reference guide on the 17 major uses and misuses of commas, available in PDF and Kindle formats. Read more about Zen Comma.