Zen Comma Rule W: Use commas to separate final descriptions that don’t refer to the immediately preceding text.
To vary sentence structure, you may put a descriptive phrase at the end of a sentence. However, readers will usually link descriptions to the closest preceding text. This is not always accurate, so we use a comma to prevent the reader from doing this.
The comma separates the description from the preceding text to show that they are not connected.
The club is disbanding, based on this letter.
In this sample, the letter states that the club is disbanding. If we leave out the comma, the sentence will state that the club is disbanding because of the letter, as seen below.
The club is disbanding based on this letter.
This indicates that the letter, or its contents, is causing the club to disband because disbanding is described by based on this letter. However, with the comma we know that disbanding is not described by based on this letter, leading to the desired interpretation.
This next sample works in a similar way.
He saw the corpse, swimming in the lake.
If we leave out the comma, the reader will think swimming in the lake is a description of the corpse, as if the corpse were swimming in the lake. Rather, swimming in the lake is a description of He. To prevent the incorrect, and odd, interpretation, we must separate the final description with a comma.
This bring us to the next type of comma uses with final descriptions.
Zen Comma Rule X: Use commas to separate non-grammatical final descriptions.
In other cases, the final description is not grammatically connected to the preceding sentence. Similar to Rule W, this can happen when the final description refers to the subject or main verb but not to the words that immediately precede the description. Consider this example:
He drove all day, unable to wait longer.
In this sample, unable to wait longer describes the subject He, not drove all day. Unlike previous examples, we cannot remove the comma and still have a grammatically correct sentence, as follows.
He drove all day unable to wait longer. (grammatically incorrect sentence)
These are a form of free modifiers, a descriptive phrase that can be moved around in the sentence. If they can’t be moved around without making the sentence confusing, they are not free modifiers and don’t need a comma. If they can be moved around, they are free modifiers, and they need commas.
Zen Comma Rule Y: Use commas to separate final coordinate expressions.
We know to put a comma between coordinate adjectives. This can also affect how we use commas with final descriptions. If the final two descriptive phrases or expressions equally describe the same thing, they are coordinate, and we separate them with a comma, as seen here:
The legislation is dead, not delayed.
To show that dead and not delayed are coordinate, we can break up the sentence as follows:
The legislation is dead. The legislation is not delayed.
Here, we see that dead and not delayed both describe legislation. We could write this in two sentences, as I have here, or we can combine them and end the sentence with both descriptions. When I combine the sentences, these two descriptions are coordinate and require a comma.
Zen Comma Rule Z: Use a comma to indicate a shift in focus at the end of the sentence.
This is fairly simple. Take a look at this sample:
The audience seemed tired, an understandable response to the boring 3-hour lecture.
The main sentence is about the audience. However, the final description is not so much about the audience as it is about the boring lecture. This is a shift in the focus of the sentence. As such, it is separated by a comma.
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