Tetrachord

In music theory, a tetrachord is a series of four notes separated by three intervals. In traditional music theory, a tetrachord always spanned the interval of a perfect fourth, a 4:3 frequency proportion —but in modern use it means any four-note segment of a scale or tone row, not necessarily related to a particular tuning system. Three modal patterns are possible:

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and the tritone:

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History

The name comes from tetra and chord. In ancient Greek music theory, tetrachord signified a segment of the greater and lesser perfect systems bounded by immovable notes ; the notes between these were movable. It literally means four strings, originally in reference to harp-like instruments such as the lyre or the kithara, with the implicit understanding that the four strings produced adjacent notes.
Modern music theory uses the octave as the basic unit for determining tuning, where ancient Greeks used the tetrachord. Ancient Greek theorists recognized that the octave is a fundamental interval but saw it as built from two tetrachords and a whole tone.

Ancient Greek music theory

theory distinguishes three genera of tetrachords. These genera are characterized by the largest of the three intervals of the tetrachord:
;Diatonic
;Chromatic
;Enharmonic
An enharmonic tetrachord has a characteristic interval that is greater than about four-fifths of the total tetrachord interval. Classically, the characteristic interval is a ditone or a major third, and the two smaller intervals are variable, but approximately quarter tones, e.g.
When the composite of the two smaller intervals is less than the remaining interval, the three-note group is called the pyknón. This is the case for the chromatic and enharmonic tetrachords, but not the diatonic tetrachord.
Whatever the tuning of the tetrachord, its four degrees are named, in ascending order, hypate, parhypate, lichanos, and mese and, for the second tetrachord in the construction of the system, paramese, trite, paranete, and nete. The hypate and mese, and the paramese and nete are fixed, and a perfect fourth apart, while the position of the parhypate and lichanos, or trite and paranete, are movable.
As the three genera simply represent ranges of possible intervals within the tetrachord, various shades with specific tunings were specified. Once the genus and shade of tetrachord are specified, their arrangement can produce three main types of scales, depending on which note of the tetrachord is taken as the first note of the scale. The tetrachords themselves remain independent of the scales that they produce, and were never named after these scales by Greek theorists.
; Dorian scale: The first note of the tetrachord is also the first note of the scale.
; Phrygian scale: The second note of the tetrachord is the first of the scale.
; Lydian scale: The third note of the tetrachord is the first of the scale.
In all cases, the extreme notes of the tetrachords, E – B, and A – E, remain fixed, while the notes in between are different depending on the genus.

Pythagorean tunings

Here are the traditional Pythagorean tunings of the diatonic and chromatic tetrachords:
Here is a representative Pythagorean tuning of the enharmonic genus attributed to Archytas:
The number of strings on the classical lyre varied at different epochs, and possibly in different localities – four, seven, and ten having been favorite numbers. Larger scales are constructed from conjunct or disjunct tetrachords. Conjunct tetrachords share a note, while disjunct tetrachords are separated by a disjunctive tone of 9/8. Alternating conjunct and disjunct tetrachords form a scale that repeats in octaves, but this was not the only arrangement.
The Greeks analyzed genera using various terms, including diatonic, enharmonic, and chromatic. Scales are constructed from conjunct or disjunct tetrachords.
This is a partial table of the superparticular divisions by Chalmers after Hofmann.

Variations

Romantic era

Tetrachords based upon equal temperament tuning were used to explain common heptatonic scales. Given the following vocabulary of tetrachords :

Tetrachord	Halfstep String
Major	2 2 1
Minor	2 1 2
Harmonic	1 3 1
Upper Minor	1 2 2

The following scales could be derived by joining two tetrachords with a whole step between:

Component tetrachords	Halfstep string	Resulting scale
Major + major	2 2 1 : 2 : 2 2 1	Diatonic major
Minor + upper minor	2 1 2 : 2 : 1 2 2	Natural minor
Major + harmonic	2 2 1 : 2 : 1 3 1	Harmonic major
Minor + harmonic	2 1 2 : 2 : 1 3 1	Harmonic minor
Harmonic + harmonic	1 3 1: 2: 1 3 1	Double harmonic scale or Gypsy major
Major + upper minor	2 2 1 : 2 : 1 2 2	Melodic major
Minor + major	2 1 2 : 2 : 2 2 1	Melodic minor
Upper minor + harmonic	1 2 2 : 2 : 1 3 1	Neapolitan minor

All these scales are formed by two complete disjunct tetrachords: contrarily to Greek and Medieval theory, the tetrachords change here from scale to scale. The 19th-century theorists of ancient Greek music believed that this had also been the case in Antiquity, and imagined that there had existed Dorian, Phrygian, or Lydian tetrachords. This misconception was denounced in Otto Gombosi's thesis.

20th-century analysis

Theorists of the late 20th century often use the term "tetrachord" to describe any four-note set when analysing music of a variety of styles and historical periods.
The expression "chromatic tetrachord" may be used in two different senses: to describe the special case consisting of a four-note segment of the chromatic scale,
or, in a more historically oriented context, to refer to the six chromatic notes used to fill the interval of a perfect fourth, usually found in descending bass lines.
It may also be used to describe sets of fewer than four notes, when used in scale-like fashion to span the interval of a perfect fourth.

Atonal usage

occasionally uses the term tetrachord to mean what he elsewhere calls a tetrad or simply a "4-element set" – a set of any four pitches or pitch classes. In twelve-tone theory, the term may have the special sense of any consecutive four notes of a twelve-tone row.

Non-Western scales

Tetrachords based upon equal-tempered tuning were also used to approximate common heptatonic scales in use in Indian, Hungarian, Arabian, and Greek musics. Western theorists of the 19th and 20th centuries, convinced that any scale should consist of two tetrachords and a tone, described various combinations supposed to correspond to a variety of exotic scales. For instance, the following diatonic intervals of one, two, or three semitones, always totaling five semitones, produce 36 combinations when joined by whole step:

Lower tetrachords	Upper tetrachords
3 1 1	3 1 1
2 2 1	2 2 1
1 3 1	1 3 1
2 1 2	2 1 2
1 2 2	1 2 2
1 1 3	1 1 3

India-specific tetrachord system

Tetrachords separated by a halfstep are said to also appear particularly in Indian music. In this case, the lower "tetrachord" totals six semitones. The following elements produce 36 combinations when joined by halfstep. These 36 combinations together with the 36 combinations described above produce the so-called "72 karnatic modes".

Lower tetrachords	Upper tetrachords
3 2 1	3 1 1
3 1 2	2 2 1
2 2 2	1 3 1
1 3 2	2 1 2
2 1 3	1 2 2
1 2 3	1 1 3

Persian

music divides the interval of a fourth differently from the Greek. For example, Al-Farabi describes four genres of the division of the fourth:

The first genre, corresponding to the Greek diatonic, is composed of a tone, a tone, and a semitone, as G–A–B–C.
The second genre is composed of a tone, a three-quarter tone, and a three-quarter tone, as G–A–B–C.
The third genre has a tone and a quarter, a three-quarter tone, and a semitone, as G–A–B–C.
The fourth genre, corresponding to the Greek chromatic, has a tone and a half, a semitone, and a semitone, as G–A–B–C.

He continues with four other possible genres "dividing the tone in quarters, eighths, thirds, half thirds, quarter thirds, and combining them in diverse manners". Later, he presents possible positions of the frets on the lute, producing ten intervals dividing the interval of a fourth between the strings:
If one considers that the interval of a fourth between the strings of the lute corresponds to a tetrachord, and that there are two tetrachords and a major tone in an octave, this would create a 25-tone scale. A more inclusive description, of the scale divisions is that of 24 quarter tones. It should be mentioned that Al-Farabi's, among other Islamic musical treatises, also contained additional division schemes, as well as providing a gloss of the Greek system, as Aristoxenian doctrines were often included.