- To join two independent clauses.
Example: "I went to the store, but I forgot my wallet."
- To separate three or more items in a series.
Example: "I bought milk, eggs, and bread at the store."
- To enclose nonrestrictive clauses.
Example: "My cousin, who is a doctor, lives in New York City."
- To set off introductory elements.
Example: "When I arrived at the party, everyone was already there."
- To prevent misreading.
Example: "Let's eat, Grandma!" ("Let's eat Grandma!" has a very different meaning.)
Semicolon
- To join two closely related independent clauses that are not joined by a conjunction.
Example: "The sky was blue; the sun was shining."
- To separate items in a series when the items are long or complex.
Example: "I saw three movies this weekend: Inception, The Dark Knight, and The Prestige; they were all excellent."
- To enclose explanatory or supplemental material.
Example: "The meeting was held in the conference room; all employees were required to attend."
- To separate two sentences that would otherwise be too closely connected.
Example: "I was surprised by his sudden departure; I thought he would stay until the end."