A silly what-if question that sounds a bit mad: I am curious as to why the letter "H" in English and some other European languages is used as a modifier to make diglyphs represent a single phoneme (ch, sh, th, dh, kh, gh etc.).
I cannot figure out what is the technical term for a letter used as modifier in diglyphs, which is half the problem.
The use of H is not universal in Latin script languages: some opt for diacritic marks and others for other letters. In Polish, the letter "Z" is used as a modifier like an H. In Italian CI is used as /ʧ/. While in Czech and Slovakian a caron diacritic is used instead of a H. While a dot above was used in Irish, but now an H due to foreign-made typewriters.
In runic alphabets, dots or marks within the letters were used. I understand that Germanic language orthographies were not translitterated from runes, but there were some borrowings, say wynn and thor, which survived in English despite French influence. So I would have predicted diacritics would have ended up being used...
A mirrored C means "con" in Latin, while I am pretty sure I read that some script used rotated printing types for some phonemes. Which also seems more logical.
Also, the use of H seems arbitrary. H is rare, but so are Q, X, Y, Z. And before printing presses, there were all sorts of symbols we don't use (9=orum, long esse etc.). Any of them could have been used.
I hope I made my logic of why I don't understand why 'H'...