[jcranmer]

> ASCII (1967) allowed for them: c BS , or , BS c ↦ ç and e BS ' or ' BS e ↦ é. Encoding ç as 63 CC A7 is not manifestly better than encoding it as 63 08 2C.

Doesn't work for ñ, since the ASCII ~ is often typeset in the middle of the box instead of in a position to appear above an 'n' character. " is a pretty poor substitute for ◌̈ though, especially when you're trying to write ï as in naïve. And then there's the æ of archæology, which doesn't work with overwriting.

I'll also point out that ç is U+00E7 in Unicode and C3 A7 in UTF-8, not 63 CC A6, since it's a precomposed character (and NFC form is usually understood to be the preferred way to normalize Unicode unless there's a reason to do something else).

[kps]

Tilde exists in ASCII because of its use as an accent. (In 1967 the non-diacritic interpretation was an overline.) The use in programming languages, and lowering to fit other mathematical operators, came later.

There was never any requirement that ‘n BS ~’ have the same appearance as ‘n’ overprinted with ‘~’, although terminals capable of making the distinction didn't appear until the 70s.

Precomposed characters aren't relevant to illustrating composition mechanisms.