Grammar Rules: Could of vs Could've

Today's grammar rule is an easy one (for most writers. I don't see this rule broken very often in the writing world.).

Correct: Could've

Incorrect: Could of

Example:
We could've gone sailing today.
It could've been a very long vacation.
The job could've turned out better.
Yes, but it could've been worse.

Authors/writers: Make sure you're on my list of authors!