Unlike an enclosed letter that has a letter meaning (see 12.3c(1)), when a single-letter abbreviation is enclosed in parentheses, quotation marks, or brackets, the punctuation should be retained. A letter sign is not required if context makes perfectly clear the meaning of a single-letter abbreviation not followed by a period. If the meaning is not clear, use a letter sign. Examples:
sizes: [s] [m] [l]
Two atoms of hydrogen (H) joined to one atom of oxygen (O) yield water.