The key signature at the beginning of a piece of music announces which sharps or flats (or neither) are in the piece, determining the key of the song. And just as the flats always occur in the same order in a key signature, so the sharps do also. Not many people realize (except musicians, of course) that the sharps and the flats are just backward to each other: in other words, the order of the flats is just reverse to the order of the sharps. Watch this 5-minute video:

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Since the sharps always occur in the same order in a key signature, it is easy to tell the key of a song simply by finding the last sharp to the right in the key signature and going up 1/2 step. So if the last sharp in the key signature is G#, the key is A major (or F# minor).