The less common use of semicolons occurs in a sentence when at least one element of a series already has commas in it. Separating the major elements of the series merely with commas could confuse the reader.

Let’s imagine that we want to write about our three favorite cities in the United States:

• Philadelphia, Pennsylvania
• Boston, Massachusetts
• Seattle, Washington

Each element in this list already has a comma in it. Although most people would have no problem separating the cities from the states, clumping them all together with only commas could make for some slow, labored reading.

Incorrect: My favorite cities in the United States are Philadelphia, Pennsylvania, Boston, Massachusetts, and Seattle, Washington.

So we use the semicolon to break the series into more meaningful chunks. Semicolons handle the big chunks and commas handle the smaller chunks within the big chunks.

Correct: My favorite cities in the United States are Philadelphia, Pennsylvania; Boston, Massachusetts; and Seattle, Washington.