In music theory, traditionally, a tetrachord (Greek: τετράχορδoν, Latin: tetrachordum) is a series of three smaller intervals that span the interval of a perfect fourth, a 4:3 frequency proportion. In modern usage a tetrachord is any four-note segment of a scale or tone row, not necessarily related to a particular system of tuning.
The term tetrachord derives from ancient Greek music theory, where it signified a segment of the Greater and Lesser Perfect Systems bounded by unmovable notes (Greek: ἑστῶτες). 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 must be contiguous.
Modern music theory makes use of the octave as the basic unit for determining tuning: ancient Greeks used the tetrachord for this purpose. Ancient Greek theorists recognised that the octave is a fundamental interval, but saw it as built from two tetrachords and a whole tone.
Ancient Greek music theory
Ancient Greek music theory distinguishes three genera (singular: genus) of tetrachords. These genera are characterised by the largest of the three intervals of the tetrachord:
- A diatonic tetrachord has a characteristic interval that is less than or equal to half the total interval of the tetrachord (or approximately 249 cents). This characteristic interval is usually slightly smaller (approximately 200 cents), becoming a whole tone. Classically, the diatonic tetrachord consists of two intervals of a tone and one of a semitone.
- A chromatic tetrachord has a characteristic interval that is greater than about half the total interval of the tetrachord, yet not as great as four-fifths of the interval (between about 249 and 398 cents). Classically, the characteristic interval is a minor third (approximately 300 cents), and the two smaller intervals are equal semitones.
- An enharmonic tetrachord has a characteristic interval that is greater than about four-fifths the total tetrachord interval. Classically, the characteristic interval is a ditone or a major third, and the two smaller intervals are quartertones.
As the three genera simply represent ranges of possible intervals within the tetrachord, various shades (chroai) of tetrachord with specific tunings were specified. Once the genus and shade of tetrachord are specified the three internal intervals could be arranged in three possible permutations.
The three permutations of the diatonic tetrachord are:
- Lydian mode
- A rising scale of two whole tones followed by a semitone, or C D E F. (same hypatē and mesē for the ancient Greeks)
- Dorian mode
- A rising scale of tone, semitone and tone, C D E♭ F, or D E F G (E to A for the ancient Greeks).
- Phrygian mode
- A rising scale of a semitone followed by two tones, C D♭ E♭ F, or E F G A (D to G for the ancient Greeks).
Here are the traditional Pythagorean tunings of the diatonic and chromatic tetrachords:
Diatonic Play (help·info) hypate parhypate lichanos mese 4/3 81/64 9/8 1/1 | 256/243 | 9/8 | 9/8 | -498 -408 -204 0 cents
Chromatic Play (help·info) hypate parhypate lichanos mese 4/3 81/64 32/27 1/1 | 256/243 | 2187/2048 | 32/27 | -498 -408 -294 0 cents
Enharmonic Play (help·info) 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. Originally, the lyre had only five to seven strings(see also the Kithara, a larger form), so only a single tetrachord was needed. 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 (a Pythagorean major second). Alternating conjunct and disjunct tetrachords form a scale that repeats in octaves (as in the familiar diatonic scale, created in such a manner from the diatonic genus), but this was not the only arrangement.
The Greeks analyzed genera using various terms, including diatonic, enharmonic, and chromatic, the latter being the color between the two other types of modes, which were seen as black and white. Scales are constructed from conjunct or disjunct tetrachords: the tetrachords of the chromatic genus contained a minor third on top and two semitones at the bottom, the diatonic contained a minor second at top with two major seconds at the bottom, and the enharmonic contained a major third on top with two quarter tones at the bottom, all filling in the perfect fourth[not in citation given][not in citation given] of the fixed outer strings. However, the closest term used by the Greeks to our modern usage of chromatic is pyknon, the density ("condensation") of chromatic or enharmonic genera.
|Didymos chromatic tetrachord||16:15, 25:24, 6:5||Play (help·info)|
|Eratosthenes chromatic tetrachord||20:19, 19:18, 6:5||Play (help·info)|
|Ptolemy soft chromatic||28:27, 15:14, 6:5||Play (help·info)|
|Ptolemy intense chromatic||22:21, 12:11, 7:6||Play (help·info)|
|Archytas enharmonic||28:27, 36:35, 5:4||Play (help·info)|
|Component Tetrachords||Halfstep String||Resulting Scale|
|Major + Major||221 2 221||Diatonic Major|
|Minor + Upper Minor||212 2 122||Natural Minor|
|Major + Harmonic||221 2 131||Harmonic Major|
|Minor + Harmonic||212 2 131||Harmonic Minor|
|Harmonic + Harmonic||131 2 131||Double Harmonic|
|Major + Upper Minor||221 2 122||Melodic Major|
|Minor + Major||212 2 221||Melodic Minor|
|Upper Minor + Harmonic||122 2 131||Neapolitan Minor|
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.
Allen Forte 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.
Tetrachords based upon Equal temperament tuning were also used to approximate common Heptatonic scales in use in Indian, Hungarian, Arabian and Greek musics. The following elements produce 36 combinations when joined by whole step:
|Lower tetrachords||Upper tetrachords|
Indian-specific tetrachord system
|Lower tetrachords||Upper tetrachords|
Since there are two tetrachords and a major tone in an octave, this creates a 25 tone scale as used in the Persian tone system before the quarter tone scale. A more inclusive description (where Ottoman, Persian and Arabic overlap), of the scale divisions is that of 24 tones, 24 equal quarter tones, where a quarter tone equals half a semitone (50 cents) in a 12 tone equal-tempered scale (see also Arabian maqam). 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.
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