The semi­colon (;) has two pri­mary uses.

1. It’s used in­stead of a con­junc­tion to com­bine two sen­tences (He did the crime; he must do the time). Don’t use a semi­colon to con­nect a sub­or­di­nate clause to a sen­tence (Since he did the crime; he must do the time). In that case, use a comma.

   In a sen­tence with a con­junc­tion, don’t use a semi­colon be­fore the con­junc­tion (He did the crime; and he must do the time). Use ei­ther a comma or no punc­tu­a­tion be­fore the con­junc­tion. Or start a new sen­tence at the conjunction.

2. A semi­colon also sep­a­rates list el­e­ments with in­ter­nal com­mas (We vis­ited Tulsa, Ok­la­homa; Flint, Michi­gan; and Pa­d­u­cah, Ken­tucky).

The colon (:) usu­ally con­nects the in­tro­duc­tion of an idea and its com­ple­tion (I own three cars: a con­vert­ible, a sedan, and a mini­van).

Semi­colons are of­ten mis­tak­enly used where a colon is cor­rect, and vice versa. Make sure you’ve got the right one.