Among the many uses of the comma is its ability to alter sentence rhythm and stress, a function that Martha Kolln says “…is probably one you hadn’t thought about before.”
In a typical sentence, emphasis is given to the predicate (or information placed at the end of a sentence). The comma can be used to interrupt normal stress and intonation patterns, redirecting the reader’s attention to key words or ideas in the sentence. This redirection can occur by placing words in front of a comma, changing their pitch and giving them additional emphasis. For example, notice how different words can be emphasized in the same sentence by moving them in front of a comma:
Original Sentence: Mina took out the garbage in the early morning.
Rewrite 1: In the early morning, Mina took out the garbage.
In the rewrite, the word morning receives more emphasis than it did in the original sentence.
Rewrite 2: Mina, in the early morning, took out the garbage.
Here again the rhythm has been altered and additional emphasis shifted to the word placed before the comma: Mina. In this case the subject, which already has a natural emphasis, gets even more.
In addition to changing intonation contours, the comma affects emphasis in another way. According to Kolln, the comma serves as a “visual signal” that essentially causes the reader to slow down and briefly hang on the word in front of it.
Here’s one more example of how placing words in front of a comma can affect the emphasis in a sentence, this one from James Joyce’s The Dead.
Original sentence: Gabriel’s eyes, irritated by the floor, which glittered with beeswax under the heavy chandelier, wandered to the wall above the piano.
Rewrite: Irritated by the floor, which glittered with beeswax under the heavy chandelier, Gabriel’s eyes wandered to the wall above the piano.
The first sentence places more emphasis on Gabriel’s eyes (and therefore the character), while the second version puts more focus on the floor and its affect on the character.