Prime mark

A prime is a short mark that leans to the right, and it means a unit: feet, minutes of arc, minutes of time. Doubled, it means the next unit down: inches, seconds. So a person is 5′ 10″ tall and a coordinate is 40° 44′ 55″ N. It is not a punctuation mark and it is not an abbreviation. It is a symbol with its own code points, U+2032 for the prime and U+2033 for the double prime, plus a triple for the rare third level.

Three different marks get used for this job and only one of them is right. The straight tick on the keyboard, U+0027, is the typewriter’s apostrophe: upright, symmetrical, and standing in for four marks at once because the machine had no room for more keys. The curly ones, U+2019 and U+201D, are closing quotation marks with comma-shaped tails, and they are what an editor with smart quotes turns your straight ticks into. The prime is the third thing: a straight stroke like the typewriter mark, but leaning, and with none of the comma-shaped tail a closing quote curls into. Set the three side by side at any size above a caption and they stop being confusable.

The confusion is the typewriter’s fault and the substitution habit is the autocorrect’s. Nearly every writing tool now converts a straight tick to a curly quote as you type, which is right for prose and exactly wrong for measurements, so 5' 10" becomes 5’ 10” without asking. The result is a height written with two closing quotation marks, which reads to a typographer roughly the way a lowercase l used as a numeral one does. If a text field in your product accepts measurements, this is worth a rule of its own: leave a tick alone when a digit sits immediately before it.

For interface work, the practical advice is to store and render the real characters rather than hope the font makes a substitute look acceptable. Most text faces draw a proper prime; those that do not fall back to something visibly different, which is at least an honest signal. And do not reach for a slanted apostrophe from an italic font, which is the traditional hot metal workaround and has no meaning in a Unicode world.