Semitone is the smallest distance between two sounds in Western music. On a piano it is the distance between two keys next to one another (counting both the white and black keys). On a guitar semitone is the distance between two adjacent frets. Two semitones form a whole tone. Only the white piano keys have their own names. Names of the black piano keys (and of the matching notes) are formed by adding either ♯ (sharp) or ♭ (flat) to the adjacent white keys. As you can see, a ♯ (sharp) raises a note by a semitone, and a ♭ (flat) lowers it respectively. As a result, a black piano key has two names.