Notes · Step 3
Semitones and the black keys
Between some lettered notes there is room for one more. The black key between C and D is a single note with two names: C♯, a semitone above C, or D♭, a semitone below D. Which name it gets depends on the music around it, not on the key itself.
A semitone is the smallest step Western music uses: from any key to the very next one, black or white. Two pairs of letters have no black key between them — E to F, and B to C — so those pairs are already a semitone apart. Count the twelve notes from one C to the next and you have every pitch a keyboard can name; then the pattern starts over.
From here the drill draws from all twelve. A note is named — sometimes with a sharp — and you find its key.