Letters in some scripts are a class of two or more characters. Usually, all letters have the same number of such case variants. Rarely, characters may be constituents of different letters within the same script. A closed set of letters, usually with a canonical sort order, makes an alphabet. Every writing system employs exactly one alphabet for each script it supports. Most writing systems only support a single script. Writing systems may have multiple systematically related orthographies, i.e. rules for combining letters into graphemes and these into words.
Any Unicode case pair is intended to be equivalent to a letter, but in some cases fails to be this. It fails in the case of Turkish <I>, because every character can only be part of a single case pair. It fails in the case of German <ß>, because a categorical error (that cannot be corrected for compatibility and stability reasons) had been made: a grapheme rule was recorded as a letter rule.
