Key signatures are a type of musical notation that indicate which key the song is to be played in.
But key signatures, despite the name, are not the same thing as key. Key signatures are simply notational devices; just as a note is the notational name for a pitch, key signatures are the notational names for keys. It is what it says it is: a signature, a simple piece of information that tips you off to the physical form (the key) to be played.
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