A Dictionary of Music and Musicians/Semitone
SEMITONE (from the Greek ἡμιτόνιον). Half a tone; the smallest interval in the ordinary musical scales. The semitone may be of different kinds, each of which has a different theoretical magnitude.
Since the invention of the diatonic scale the natural interval of the fourth has been subdivided artificially into two tones and a semitone. In the ancient Greek time the two tones were both what are now called major tones, and the hemitone had a magnitude determined by the difference between their sum and the fourth: but when harmony began to prevail, one of the tones was diminished to a minor tone, and this gave the modern semitone a little greater value. The semitone, so formed, as belonging to the diatonic scale (from B to C, or from E to F for example) is called a diatonic semitone.
The introduction of chromatic notes gave rise to a third kind of semitone, as from C to C♯ or from G to G♭; this is called a chromatic semitone and has a less magnitude than the diatonic one.
Finally came the great simplification of music by dividing the octave into twelve equal intervals, each of which was called a mean semitone; thus abolishing practically the difference between the diatonic and the chromatic values. A semitone may now be considered, in practical music, as simply the interval between the sounds given by any two adjoining keys on a well-tuned piano.
The relations between the theoretical magnitudes of the different kinds of semitones are about as follows:—If we represent the magnitude of a mean semitone by 25, the true magnitude of a diatonic semitone will be about 28; of a chromatic semitone about 18; and of the ancient Greek hemitone about 23.