Usage and Grammar
Q. Parenthetical material is usually invisible to the grammar of the rest of the sentence, so should it be “a” or “an” in the phrase “a (appropriate) joke”?
A. Although almost anything can be placed inside them (Is it Friday yet?), parentheses don’t make the words they enclose literally invisible. Readers are still obliged to read what’s inside along with the rest of the text. So write “an (appropriate) joke,” which will spare us from reading “a appropriate joke,” a phrase that, stylistically speaking, would be inappropriate.