Use parentheses to enclose words or figures that clarify or are used as an aside.
I expect five hundred dollars ($500).
He finally answered (after taking five minutes to think) that he did not understand the question.
Commas could have been used in the above example. Parentheses show less emphasis or importance.
Em dashes, which could also have been used instead of parentheses, show emphasis.
Use full parentheses to enclose numbers or letters used for listed items.
We need an emergency room physician who can (1) think quickly, (2) treat patients respectfully, and (3) handle complaints from the public.
Periods go inside parentheses only if an entire sentence is inside the parentheses.
Please read the analysis (I enclosed it as Attachment A.).
Please read the analysis. (I enclosed it as Attachment A.)
Please read the analysis (Attachment A).