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Q. I’m proofreading a math textbook that ends a sentence with “25 in.” followed by a superscript 2, denoting square inches. (Our math textbooks do not use “sq. in.”) There is a period after “in” and then another period after the superscript: 25 in.2. My gut says to eliminate the second period. What say you?
A. CMOS is silent on the issue of punctuation after a superscript following a period, and it doesn’t seem to be addressed in the science-related reference books at hand here, so assuming your house style is in. and in.2 (with the period), you could look at the situation in two different ways in order to choose your own style:
(1) You could see it as similar to when an abbreviation appears at the end of a parenthesis that ends a sentence, in which case a period appears both at the end of the abbreviation and at the end of the sentence:
Parenthesis style: The answer is doubled (25 in.2).
Applied to a superscript: The answer is 25 in.2.
(2) Or you could see it as similar to when an abbreviation ends a quotation, in which case only one period appears at the end:
Quotation mark style: The answer is “25 in.2”
Applied to a superscript: The answer is 25 in.2
A math professor we consulted on campus who is also the author of a geometry textbook had not run into this issue before, but she thought both suggestions were reasonable.