|Which is correct?
When it comes to compliance, make sure all your ____________________.
a. i’s are dotted and t’s are crossed
b. is are dotted and ts are crossed
Explanation: Pluralization of single letters is the ONLY time that apostrophes are used to make something plural. The ONLY time.
Check out the Apostrophe Abuse blog to see the type of mockery you could make yourself subject to if you break the rule.
Grammar Girl covers pluralization of single letters in her post about The Apostrophe Catastrophe.