Half Steps, Whole Steps, and Accidentals
A half step (semitone) is the smallest interval in standard Western music: the distance from any key to the immediately adjacent key on the piano, whether black or white. A whole step (whole tone) equals two half steps. These two building blocks define the internal structure of all scales and intervals.
Accidentals alter a note's pitch from its natural (white-key) position. A sharp (♯) raises a pitch by one half step; a flat (♭) lowers it by one half step; a natural (♮) cancels a previous sharp or flat. A double sharp (𝄪) raises by two half steps; a double flat (𝄫) lowers by two half steps. Accidentals apply to every subsequent occurrence of that note within the same measure unless cancelled.
On the keyboard, most white-key pairs separated by a black key are a whole step apart (C–D, D–E, F–G, G–A, A–B). The pairs E–F and B–C have no black key between them and are therefore a half step apart naturally. This asymmetry is why not every major scale step looks the same on the keyboard, and why sharps and flats are essential for building scales starting on any note.