In Western music theory, a semitone (also called a half step or half tone) is one of two parts into which a whole tone is divided. Semitones are the smallest intervals commonly harmonically and melodically used in Western music, and are among the most dissonant intervals when sounded harmonically. A progression of semitones forms a chromatic scale, which informs the layout of many fixed-pitch instruments. For example, C is adjacent to D♭ on a keyboard, so the interval between them is a semitone. In musical notation, two types of semitones are distinguished. A diatonic semitone, or minor second, arises between two notes encompassing different staff positions, e.g. from C to D♭. A chromatic semitone, or augmented unison, arises between two notes at the same staff position, e.g. from D♭ to D♮. The sum of these two intervals, the interval from C to D, is a whole tone, or major second. The modern tuning system of 12-tone equal temperament divides the octave into twelve equal semitones, each with a frequency ratio of the twelfth root of two (100 cents). In this system, a given interval can be represented by a specific number of semitones from any starting note. For example, two semitones may represent a major second, four semitones a major third, and seven semitones a perfect fifth (see: Interval (music) § Number, for more on interval terminology). In tuning systems derived from stacking perfect fifths, i.e. Pythagorean tuning and the related meantone temperaments, diatonic semitones all have the same size, as do chromatic semitones, but the two sizes are distinct. For example, they are not the same size in Pythagorean tuning, where the diatonic semitone is distinguished from the larger chromatic semitone, or in quarter-comma meantone temperament, where the diatonic semitone is larger instead. The condition of having semitones is called hemitonia; that of having no semitones is anhemitonia. A musical scale or chord containing semitones is called hemitonic; one without semitones is anhemitonic.
Minor second
The minor second (m2 or -2) occurs in the major scale, between the third and fourth degree, (mi (E) and fa (F) in C major), and between the seventh and eighth degree (ti (B) and do (C) in C major). It is also called the diatonic semitone because it occurs between steps in the diatonic scale. Its inversion is the major seventh (M7 or Ma7).
Melodically, this interval is very frequently used, and is of particular importance in cadences. In the authentic and deceptive cadences it appears as a resolution of the leading tone to the tonic. In the plagal cadence, it appears as the falling of the subdominant to the mediant. It also occurs in many forms of the imperfect cadence, wherever the tonic falls to the leading tone.
Harmonically, the interval usually occurs as some form of dissonance or a nonchord tone that is not part of the functional harmony. It may also appear in inversions of a major seventh chord, and in many added tone chords.
In unusual situations, the minor second can add a great deal of character to the music. For instance, Frédéric Chopin's Étude Op. 25, No. 5 opens with a melody accompanied by a line that plays fleeting minor seconds. These are used to humorous and whimsical effect, which contrasts with its more lyrical middle section. This eccentric dissonance has earned the piece its nickname: the "wrong note" étude. This kind of usage of the minor second appears in many other works of the Romantic period, such as Modest Mussorgsky's "Ballet of the Unhatched Chicks" from Pictures at an Exhibition. More recently, the music to the movie Jaws exemplifies the minor second.
Augmented unison
The augmented unison, augmented prime, or chromatic semitone is the interval between two notes on the same staff position (same letter name) that differ by one chromatic alteration. For example, the interval between B♭ and B♮, or between C♮ and C♯, is an augmented unison.
The term, in its French form unisson superflu, appears to have been coined by Jean-Philippe Rameau in 1722, who also called this interval a minor semitone (semiton mineur). Historically, this interval, like the tritone, is described as being "mi contra fa" and was associated with the "diabolus in musica" (Latin for 'the Devil in music').
Melodically, an augmented unison very frequently occurs when proceeding to a chromatic chord, such as a secondary dominant, a diminished seventh chord, or an augmented sixth chord. Its use is also often the consequence of a melody proceeding in semitones, regardless of harmonic underpinning, e.g. D, D♯, E, F, F♯. (Restricting the notation to only minor seconds is impractical, as the same example would have a rapidly increasing number of accidentals, written enharmonically as D, E♭, F♭, G, A).
Harmonically, augmented unisons are quite rare in tonal repertoire. In the example to the right, Liszt wrote an E♭ against an E♮ in the bass. Here E♭ was preferred to a D♯ to make the tone's function clear as part of an F dominant seventh chord, and the augmented unison is the result of superimposing this harmony upon an E pedal point. In addition to this kind of usage, harmonic augmented unisons are frequently written in modern works involving tone clusters, such as Iannis Xenakis' Evryali for solo piano.
Diminished unison The term diminished unison or diminished prime is also found occasionally. It is found once in Rameau's writings, for example, as well as subsequent French, German, and English sources. Other sources reject the possibility or utility of the diminished unison on the grounds that any alteration to the unison increases its size, thus augmenting rather than diminishing it. The term is sometimes justified as a negative-numbered interval, and also in terms of violin double-stopping technique on analogy to parallel intervals found on other strings. Some theoreticians make a distinction for this diminished form of the unison, stating it is only valid as a melodic interval, not a harmonic one.
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