Semi-colons are pretty simple punctuation marks; they only do two things.
1. Semi-colons join two independent clauses (complete sentences):
I didn’t like her cooking; I just ate it to be polite.
“I didn’t like her cooking” is a complete sentence, as is “I just ate it to be polite.” Therefore, they can be joined with a semi-colon. Note that complete sentences can begin with “therefore” and “however.”
2. Semi-colons are used between items in a series if at least one of the items contains appositive material (explanatory words set off with commas):
We invited my brother, Bill; my sister, Jeanne; and my cousin, Pat.
Each “item” in this series contains a word set off by commas:
- my brother, Bill
- my sister, Jeanne
- my cousin, Pat
Therefore, we know that we invited three people. If we had only used commas, the list of people we invited could be confusing:
- my brother
- my sister
- my cousin
The semi-colon is used to keep the items in a list separate, so that it’s clear who was invited.
One last semi-colon rule: place them outside quotation marks.