Q. Periods always go inside quotation marks. I have been told that the exception is when the matter within the quotation marks is a number or single letter. For example: The figure is impressed “1”. Please tell me if I have been misinformed!
A. You are somewhat misinformed. The point is to move the period when it’s essential to make clear that the period is not included in the quoted matter, no matter how many letters or numbers are quoted:
Type in the code “W1.GH.748”.
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Chicago style spells out the numbers one through one hundred, so a single digit is not likely to appear in quotation marks, but a single letter may be quoted with a period to no ill effect:
He demanded to know who ate the Twinkie, and she answered, “I.”
Asked to give a middle initial, she replied, “X.”