I knew the symbol properly used for a foot (measurement of length), as in 5′, was called a “prime,” and I occasionally vaguely wondered why, but it’s one of those things I never got around to investigating. Now I have, and here’s what Wikipedia has to say:
The name “prime” is something of a misnomer. Through the early part of the 20th century, the notation x′ was read as “x prime” not because it was an x followed by a “prime symbol”, but because it was the first in the series that continued with x″ (“x second”) and x‴ (“x third”). It was only later, in the 1950s and 1960s, that the term “prime” began to be applied to the apostrophe-like symbol itself. Although it is now more common to pronounce x″ and x‴ as “x double prime” and “x triple prime”, these are still sometimes pronounced in the old manner as “x second” and “x third”.
Mind you, this is followed by “,” but it’s plausible enough I’m willing to accept it provisionally. If anyone knows of a more dependable resource on the subject, by all means speak up. And remember, it’s not 5’10″ (with apostrophes or end quotes), it’s 5′10″!