Past tense of 'flat': made flat or level; in music, lowered by a semitone (preceded by a flat symbol).
From 'flat' (verb) + '-ed' past tense suffix. Also used as an adjective in music contexts to describe a note that is lowered in pitch.
In music, every note can be 'flatted' (lowered a half-step) or 'sharped' (raised a half-step)—so C-flat is just below C, which is a genius system that lets musicians describe any pitch using just note names and these two operations!
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