White keys are the natural notes C D E F G A B, then the pattern repeats. The two-then-three black-key groups always start on C.

Each black key has two names. C♯ (raise C) equals D♭ (lower D) — same pitch, different spelling. These pairs are called enharmonic.

There is no black key between E–F or B–C. Those neighbours are already only a semitone apart.

The octave number increases at every C. Middle C is C4. Concert pitch A4 = 440 Hz, the standard tuning reference.

Each semitone up multiplies the frequency by 2^(1/12). Twelve semitones double it — one octave higher.