What do flats (♭) and sharps (♯) mean?
A flat (♭) lowers a note by one — one key to the left; a sharp (♯) raises it one key to the right. The same black key can carry either name: E♭ and D♯ are one key.
Only seven letters name the notes, A to G, but there are twelve keys in an octave, so the five black keys borrow their names from the white keys beside them. A sharp (♯) means 'the key just above this letter'; a flat (♭) means 'the key just below.' That gives every black key two names — the one between D and E is D♯ coming up from D and E♭ coming down from E — and which name gets used depends on the . Blues writing spells its lowered notes as flats (E♭, the ♭5 in A blues) because they are thought of as notes pulled down; the synth keyboard labels the same key D♯. Both are correct; it is one key with two names.