Mr A Music Place
http://mramusicplace.net/2016/02/25/do-you-really-know-what-a-key-signature-is/
I’m fairly certain that if I asked a room full of music teachers to tell me what a key signature is, nearly everyone would tell me something akin to “it is an indication of which pitches will be sharped or flatted throughout the piece of music.” As accurate as that is, it is also simplistic enough to allow us to overlook some details
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I Write The Music 