In music, a flat (♭) lowers a note by a semitone and a sharp (♯) raises it by a semitone: B♭ sits just below B, and F♯ sits just above F. The same logic applies to performance — a singer who is flat is under the correct pitch, and one who is sharp is above it. Flat goes down, sharp goes up.
The core difference
Flat and sharp are accidentals: symbols that shift a written note by one semitone, the smallest step on a piano keyboard. The flat sign ♭ pushes the note down; the sharp sign ♯ pulls it up. A third symbol, the natural (♮), cancels either one and returns the note to its unaltered pitch.
How they work on the page
Written before a notehead, an accidental applies to that note for the remainder of the bar. Written at the start of a stave, a group of them forms the key signature and applies for the whole piece: two sharps means every F and C is played sharp unless marked otherwise, which is the key of D major.
- On a keyboard, the black key immediately to the right of C is C♯; the black key immediately to the left of B is B♭.
- In speech, players say "B flat" and "F sharp" — the modifier follows the letter, unlike the written symbol.
How they work in tuning
Used of a performance rather than a note, the words describe error. "The second violin was flat in the slow movement" means the pitch sagged below where it should have been. "The choir went sharp in the last verse" means it crept upwards, which is what a nervous or over-energetic ensemble tends to do.
The memory trick
A sharp object points upwards and pricks you; sharp raises the pitch. Something flat lies down on the floor; flat lowers it. The shapes cooperate too — the ♭ sign is a small b, and lowercase b sits low.
The edge case worth knowing
Some sharps and flats name the same sound. F♯ and G♭ are played by the identical piano key; so are C♯ and D♭. These are called enharmonic equivalents, and which name you use depends on the key you are in — a piece in E major spells the note D♯, while one in A♭ major spells it E♭. Getting this wrong is not a wrong note, but it makes music harder to read.
Two further quirks catch beginners out. First, E♯ and B♯ exist, and they are simply F and C — there is no black key between E and F or between B and C. Second, in string playing and singing, where pitch is continuous rather than fixed, players often make a sharpened note slightly higher than its flat equivalent, so F♯ and G♭ are not always literally the same sound.
