A symbol in music that denotes a note should be lowered by one half step. You can remember flat means lower by thinking of a flat tire. When a car has a flat tire, it gets lower to the ground.
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Example: Bb means the pitch would be a B lowered by one half step, or down one fret on the guitar.
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The term flat is also used when tuning to indicate the pitch of a note is slightly too low to be in tune.