No, you do, you really do, though a sigla might be for you an unknown known (a real category, though one missed out by Donald Rumsfeld in his famous speech).
A sigla is a list, with explanations, of all the symbols to be found in a book. If the compiler of the book has any sense it'll be at the beginning where people will find it before getting bewildered by squiggles like ß, Ɣ or ᵿ.
Couldn't do without them, could we?
Word To Use Today (though probably not out loud because that might be showing off): sigla. Sigla is a word like agenda, that is, it's a Latin plural, so the word siglas, actually therefore a plural of a plural, shouldn't have a Latin-looking ae ending. The word sigla is the plural of the Latin siglum, which is a diminutive of signum, sign.