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.

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
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:
Diatonic
hypate parhypate lichanos mese
4/3 81/64 9/8 1/1
| 256/243 | 9/8 | 9/8 |
-498 -408 -204 0 cents
Chromatic
hypate parhypate lichanos mese
4/3 81/64 32/27 1/1
| 256/243 | 2187/2048 | 32/27 |
-498 -408 -294 0 cents
Here is a representative Pythagorean tuning of the enharmonic genus attributed to Archytas:
Enharmonic
hypate parhypate lichanos mese
4/3 9/7 5/4 1/1
| 28/27 |36/35| 5/4 |
-498 -435 -386 0 cents
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.
Didymos chromatic tetrachord4:310:916:151:1
Eratosthenes chromatic tetrachord4:310:920:191:1
Ptolemy soft chromatic4:310:928:271:1
Ptolemy intense chromatic4:38:722:211:1
Archytas enharmonic4:39:728:271:1

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 :
TetrachordHalfstep String
Major2 2 1
Minor2 1 2
Harmonic1 3 1
Upper Minor1 2 2

the following scales could be derived by joining two tetrachords with a whole step between:
Component TetrachordsHalfstep StringResulting Scale
Major + Major2 2 1 : 2 : 2 2 1Diatonic Major
Minor + Upper Minor2 1 2 : 2 : 1 2 2Natural Minor
Major + Harmonic2 2 1 : 2 : 1 3 1Harmonic Major
Minor + Harmonic2 1 2 : 2 : 1 3 1Harmonic Minor
Harmonic + Harmonic1 3 1 : 2 : 1 3 1Double Harmonic Scale or Gypsy Major
Major + Upper Minor2 2 1 : 2 : 1 2 2Melodic Major
Minor + Major2 1 2 : 2 : 2 2 1Melodic Minor
Upper Minor + Harmonic1 2 2 : 2 : 1 3 1Neapolitan 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 later 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 describes 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 tetrachordsUpper tetrachords
3 1 13 1 1
2 2 12 2 1
1 3 11 3 1
2 1 22 1 2
1 2 21 2 2
1 1 31 1 3

Indian-specific tetrachord system

See also Carnatic rāga and Hindustani classical music.
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 tetrachordsUpper tetrachords
3 2 13 1 1
3 1 22 2 1
2 2 21 3 1
1 3 22 1 2
2 1 31 2 2
1 2 31 1 3

Persian

music divides the interval of a fourth differently than the Greek. For example, Al-Farabi describes four genres of the division of the fourth:
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 treatises, also contained additional division schemes as well as providing a gloss of the Greek system as Aristoxenian doctrines were often included.

Compositional forms

The tetrachord, a fundamentally incomplete fragment, is the basis of two compositional forms constructed upon repetition of that fragment: the complaint and the litany.
The descending tetrachord from tonic to dominant, typically in minor, had been used since the Renaissance to denote a lamentation. Well-known cases include the ostinato bass of Dido's aria When I am laid in earth in Henry Purcell's Dido and Aeneas, the Crucifixus in Johann Sebastian Bach's Mass in B minor, BWV 232, or the Qui tollis in Mozart's Mass in C minor, KV 427, etc. This tetrachord, known as lamento, has been used until today. A variant form, the full chromatic descent, has been known as Passus duriusculus in the Baroque Figurenlehre.
There exists a short, free musical form of the Romantic Era, called complaint or complainte or lament. It is typically a set of harmonic variations in homophonic texture, wherein the bass descends through some tetrachord, possibly that of the previous paragraph, but usually one suggesting a minor mode. This tetrachord, treated as a very short ground bass, is repeated again and again over the length of the composition.
Another musical form, of the same time period, is the litany or litanie, or lytanie. It is also a set of harmonic variations in homophonic texture, but in contrast to the lament, here the tetrachordal fragment – ascending or descending and possibly reordered – is set in the upper voice in the manner of a chorale prelude. Because of the extreme brevity of the theme and number of repetitions required, and free of the binding of chord progression to tetrachord in the lament, the breadth of the harmonic excursion in litany is usually notable.