Prime Marks
The straight vertical marks (′ and ″) that denote feet, inches, minutes, and seconds — often confused with quotation marks.
Prime marks are the straight vertical marks used to indicate units: a single prime (′) for feet, arcminutes, or minutes of time; a double prime (″) for inches, arcseconds, or seconds. They are typographically distinct from both straight and curly quotation marks, though in casual writing all four are frequently confused.
The confusion has a mechanical origin. Typewriters had only one key for a vertical mark that could stand in for opening quote, closing quote, apostrophe, prime, and double prime. Early digital text inherited this shortcut. Modern typesetting distinguishes them: 'straight quotes' (" ") are neutral quotation marks, 'curly quotes' (“ ”) are the true typographic quotation marks, and prime marks (′ ″) are a separate glyph reserved for measurement.
The difference is visible when the fonts are set well: prime marks lean slightly to the right and have no curl or ball terminal. In a properly typeset text, 6′2″ (six feet two inches) uses primes, not quotation marks or apostrophes, and any editor who cares about typography will make the substitution.
Common mistake: writing '6' 2"' with straight quotation marks instead of prime marks and letting the auto-correct convert them to curly quotes, which produces the visually odd '6' 2"'. The rule: units take primes; speech and quotation take quotation marks; possession and contraction take the apostrophe.
“The tower stands at 45°25′30″ N.”
Keep reading the glossary
Curly typographic quote marks ('smart') versus the vertical marks of typewriters and code ('straight').
The punctuation that signals direct speech or a quoted phrase — single or double, depending on locale.
A national style difference — double in American English, single in much of British English.
The em-dash convention for attributing a freestanding quote (— Author).