What is a clear and concise definition of a punctuation key?
Punctuation is defined by American Heritage Dictionary (on Dictionary.com) as "the standard marks and signs in writing and printing" that are used to clarify meaning by designating the separations of thoughts and concepts into sentences and their constituent clauses and phrases and other units of meaning.
A "key" for punctuation may be one of two things. First, a punctuation key may be the system for explaining the use and function of punctuation marks, analogous to a map key. Second, a punctuation key may be one of the many marked parts or levers depressed in operating a computer terminal or a typewriter, which is marked with one of the various symbols of "standard marks and signs" used to designate units of sentence construction and clarify meaning.