Not to detract from the substance of your post, but the unshifted keyboard key pretty much universally generates an ASCII apostrophe (U+0027, '), not a prime (U+2032, ′, as used for feet and minutes, and while I'm babbling, the addition of a second prime for seconds is not a coincidence).
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ASCII 1967, in conjunction with its European counterpart ECMA-6, permitted the visual appearance of some characters, including the apostrophe, to be modified so that they could be used as accents when overstruck¹:
' " , ^ ` ~
ˊ ¨ ¸ ˆ ˋ ˉ (using the modern Unicode spacing modifiers)
Later versions of ECMA-6 suggested that the accent interpretation be used only when the character was actually overstruck:
In the 7-bit character set, some printing symbol may be
designed to permit their use for the composition of ac-
cented letters when necessary for general interchange of
information. A sequence of three characters, comprising
a letter, BACKSPACE and one of these symbols, is needed
for this composition; the symbol is then regarded as a dia-
critical sign. It should be noted that these symbols take
on their diacritical significance only when they precede or
follow the character BACKSPACE; for example, the symbol
corresponding to the code combination 2/7 normally has the
significance of APOSTROPHE, but becomes the diacritical
sign ACUTE ACCENT when preceded or followed by the character
BACKSPACE.²
Although this was fine for the printing teletypes in use when ASCII was first designed, early video terminals were too dumb to handle overstrikes, so it never caught on.
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ASCII 1967, in conjunction with its European counterpart ECMA-6, permitted the visual appearance of some characters, including the apostrophe, to be modified so that they could be used as accents when overstruck¹:
Later versions of ECMA-6 suggested that the accent interpretation be used only when the character was actually overstruck: Although this was fine for the printing teletypes in use when ASCII was first designed, early video terminals were too dumb to handle overstrikes, so it never caught on.¹Revised American Standard Code for Information Interchange, from http://www.wps.com/J/codes/Revised-ASCII/index.html
²ECMA-6: 7-Bit coded Character Set, 4th Edition, August 1973, from http://www.ecma-international.org/publications/files/ECMA-ST...
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