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A letter of a formal language is a more or less arbitrary symbol whose interpretation depends on the specific context.
In building a formal language, letters are considered to be the undefined terms of said language.
An important part of assigning semantics to a formal language is to provide an interpretation for its letters.
The vocabulary of a formal language is the collection of all of its letters.
As such, it forms part of the alphabet of this language.
Each signature for the language may have a different vocabulary.
However, different signatures only have different members of the respective classes of symbols comprising the alphabet, rather than introducing new such classes.