6. Grammar/Mechanics Checkup 9: Semicolons and Colons You will use commas and periods most frequently in your business sentences. However, a good writer should know how to use colons and semicolons as well. These two punctuation marks might look similar, but they serve different purposes. Determine what punctuation rule is illustrated by the following sentence. At our company retreat, I met employees from Tempe, Arizona; Denver, Colorado; and Honolulu, Hawaii. Use a colon to separate items in a series when one or more of the items contain internal commas. Use a semicolon to separate items in a series when one or more of the items contain internal commas. Use a colon to join closely related independent clauses.

The punctuation rule that is illustrated in the sentence given above in the question is "use a semicolon to separate items in a series when one or more of the items contain internal commas".

As can be seen in the sentence "At our company retreat, I met employees from Tempe, Arizona; Denver, Colorado; and Honolulu, Hawaii.", there are internal commas that separate the several items listed in the sentence. We have a comma separating "Tempe from Arizona, suggesting a specific place in Arizona where employees were met, i.e. Tempe. Same applies to the rest places mentioned in the sentence, with a semicolon separating each set of employees met in each area.