Diatonic and chromatic

Diatonic (Greek: διατονική) and chromatic (Greek: χρωματική) are terms in music theory that are most often used to characterize scales, and are also applied to musical instruments, intervals, chords, notes, musical styles, and kinds of harmony. They are very often used as a pair, especially when applied to contrasting features of the common practice music of the period 1600–1900.

These terms may mean different things in different contexts. Very often, diatonic refers to musical elements derived from the modes and transpositions of the "white note scale" C–D–E–F–G–A–B. In some usages it includes all forms of heptatonic scale that are in common use in Western music (the major, and all forms of the minor).Chromatic most often refers to structures derived from the chromatic scale, which consists of all semitones. Historically, however, it had other senses, referring in Ancient Greek music theory to a particular tuning of the tetrachord, and to a rhythmic notational convention in mensural music of the 14th through 16th centuries.

In ancient Greece there were three standard tunings (known by the Latin word genus, plural genera) of a lyre. These three tunings were called diatonic,chromatic, and enharmonic, and the sequences of four notes that they produced were called tetrachords ("four strings"). A diatonic tetrachord comprised, in descending order, two whole tones and a semitone, such as A G F E (roughly). In the chromatic tetrachord the second string of the lyre was lowered from G to G♭, so that the two lower intervals in the tetrachord were semitones, making the pitches A G♭ F E. In the enharmonic tetrachord the tuning had two quarter tone intervals at the bottom: A G F E (where F is F♮ lowered by a quarter tone). For all three tetrachords, only the middle two strings varied in their pitch.

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