halfstep

A half step, also called a semitone, is the smallest interval used in most Western tonal music. It is the pitch difference between two adjacent notes, such as C and C-sharp or E and F. In standard equal temperament, a half step corresponds to a frequency ratio of about 2^(1/12) (approximately 1.05946). There are 12 semitones in an octave.

In terms of categorization, semitones can be chromatic or diatonic. A chromatic semitone occurs between notes that differ by an accidental (for example, C and B-sharp), while a diatonic semitone occurs between adjacent degrees of a diatonic scale (for example, E and F in the C major scale). In equal temperament, both types are the same size, though historical tunings treated them slightly differently.

The half step is a fundamental unit for building scales, melodies, and harmonies. It defines the pattern of intervals within scales (for example, the major scale has a sequence of whole steps and half steps: whole, whole, half, whole, whole, whole, half). Semitones are also central to voice leading and chord spelling, where accidentals raise or lower notes by a half step to form altered tones, chromatic chords, or modal flavors.

Tuning systems and historical practice can alter the exact size of a semitone. In just intonation or meantone temperament, chromatic and diatonic semitones may differ in size. Beyond common Western music, many musical traditions employ microtones that are smaller than a semitone, making the concept of a single half step less universal.