Diatonicism
System of the seven basic notes of a scale.
What it means
Diatonicism designates a system of seven notes per octave in which five whole-tone steps and two semitone steps are distributed so that the semitones lie as far apart as possible. On the keyboard this corresponds to the white keys. From this uneven distribution everything else arises: each degree has a different distance from its neighbours, and therefore each has a character of its own within the scale.
Musical use
This unevenness is the reason why music can have a centre. In a scale made of nothing but equal steps — the whole-tone scale or the chromatic scale — no note is singled out from the others; in a diatonic one, by contrast, the position of the semitones produces a gradient: the leading tone pushes toward the root, the dominant aims at the tonic. Cadence and functional harmony rest on this.
Depending on which degree one begins on, different modes arise from the same pool of notes — from C the major scale, from A natural minor, from D Dorian, and so on. The medieval church modes and the later major–minor tonality are thus manifestations of the same diatonic stock. Historically it was built up from tetrachords and hexachords, later derived from the stacking of fifths.
Diatonicism is to be distinguished from chromaticism, which uses notes outside this pool: chromatic notes do not cancel diatonicism but colour it — their effect rests on a diatonic frame remaining audible.
The diatonic system is the basis of European music from the Middle Ages to the present. Church modes, major, and minor all draw on the same pool of seven basic notes, which is mapped on the white keys of the piano and can be arranged in fifths. Its opposite term is chromaticism, which introduces intermediate degrees.
Performance practice
Do not practise scales as a finger exercise but with an eye to the semitone places: where do they lie, and where do they push? Sing the degrees and take care that the leading tone moves close to the root. When working up a piece, it helps to check first which notes belong to the scale and which do not — it is the exceptions that carry the course of the music.