Tetrachord
A four-note series within a fourth.
What it means
A tetrachord is a sequence of four notes that fills the span of a fourth. The outer notes stand fixed a fourth apart; the two inner ones can be placed differently — from this arise different types of tetrachord. The concept comes from ancient Greek music theory and remained, beyond the Middle Ages, a means of describing scales.
Musical use
The major scale can conveniently be assembled from two identically built tetrachords: whole-whole-half, a whole tone as a link, then whole-whole-half again. This breakdown is more than a mnemonic — it explains the circle of fifths. The upper tetrachord of a major scale is at the same time the lower tetrachord of the key a fifth higher, which is why exactly one new accidental is added with every step of a fifth.
A distinction is made between conjunct and disjunct linking: with the former, two tetrachords share a note, with the latter a whole tone separates them. The major scale is built disjunctly.
Ancient theory knew three genera, which differ in the position of the inner notes — the diatonic with whole and half steps, the chromatic with two narrow steps and a major third, the enharmonic with even smaller intervals. For the practical music of the West, the diatonic remained authoritative.
The tetrachord is to be distinguished from the hexachord, which comprises six notes and was the basis of medieval solmization.
The tetrachord was the basic unit of ancient Greek music theory. The major scale consists of two identically built tetrachords separated by a whole tone. The type of tetrachord, diatonic, chromatic, or enharmonic, depended on the position of the inner notes. In the Baroque, the descending tetrachord from the tonic to the dominant is a well-known lament bass pattern, which also occurs filled in chromatically and forms the basis of many lament scenes.
Performance practice
Use the breakdown when learning keys: whoever knows the upper tetrachord of a scale already has the lower one of the next key a fifth away. Practise scales tetrachord by tetrachord too, with deliberate attention to the half step at the upper end of each group.