Subscribe to The Chicago Manual of Style Online
Usage and Grammar
Q. Hello there, I am usually pretty confident about sorting out punctuation, but recently I encountered some information set out as follows.
Ref. no.: xxx
I know the colon and the period look silly next to each other, but I guess I just need to know which one to remove and why.
A. I realize we’ve been hammering lately on not having two periods or two punctuation marks in a row, so I don’t blame you for being confused, but the dot in “no.” is not part of the sentence or phrase punctuation; it’s part of the abbreviation. You have to have it. And the colon has its own job to do, so you have to have it as well. A period can do double duty: if an abbreviation ends a sentence, you don’t need two dots. But the period is not flexible enough to convey the meaning of a colon.