Because I am a freak, I suffer some considerable anxiety about the proper way of dealing with complete sentences inside of parenthetical enclosures. Most of this is purely aesthetic. For example, I hate constructs such as “XX XX (XX XX.).” I just hate the period-parenthesis-period. Or anything similar: “XX XX (XX XX!).”. So I will never put a complete sentence inside an inner-sentence parenthetical enclosures at the very end of a sentence. But before I go any further, we should look at what The Book says.

According to The Chicago Manual of Style 15th Edition:

I don’t think I have ever included more than one sentence inside a parenthetical enclosure in a sentence. Certainly, doing so outside a sentence is done all the time. In fact, one or more paragraphs are sometimes put inside a parenthetical enclosure, although it seems to me that such things should be put into footnotes. What’s more, I feel no compunction about putting rather complex sentences (including: colons and semicolons) inside inner-sentence parenthetical enclosures. But care must be taken.

It is a matter of opinion, but any writer must ask if a parenthetical statement is so long that it causes the sentence on one side of it to lose contact with that on the other. If that’s the case, the whole construction must be reevaluated. There are always alternatives. Often, I find that things I initially put inside parenthetical enclosures really want to be independent sentences. This is especially true when what’s inside is very long. Consider putting it in its own paragraph—without parentheses. Often what at first seems parenthetical is not.

The quotation from The Book seems to indicate that these inner-sentence parenthetical enclosures should never end with a period. However, there is no admonition against any other full stops and there is no guidance whatsoever about capitalization. This is where my biggest concerns lie. These are the rules I’ve developed for complete sentences inside of inner-sentence parenthetical enclosures:

• If it is at the end of a sentence, put it outside the sentence. If this makes the meaning confused, rewrite the paragraph and find something that does work.
• Otherwise, capitalize the first letter in the sentence.

What I often see are parenthetical enclosures that end with question marks or exclamation points that do not capitalize the first word of the sentence. This is certainly what The Book seems to dictate. But—because I’m a freak—it drives me crazy. And I think it is confusing, because these are full stops. But I don’t know of anyone else formatting inner-sentence parenthetical enclosures in my way. But they should. Anything else is inconsistent and just odd.