Tetrachord

In music theory, a tetrachord (Greek: τετράχορδoν, Latin: tetrachordum) 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. In music theory, a tetrachord (Greek: τετράχορδoν, Latin: tetrachordum) 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. The name comes from tetra (from Greek—'four of something') and chord (from Greek chordon—'string' or 'note'). In ancient Greek music theory, tetrachord signified a segment of the greater and lesser perfect systems bounded by unmovable notes (Greek: ἑστῶτες); the notes between these were movable (Greek: κινούμενοι). 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 (i.e., conjunct) 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 distinguishes three genera (singular: genus) of tetrachords. These genera are characterized by the largest of the three intervals of the tetrachord: Whatever the tuning of the tetrachord, its four degrees are named, in ascending order, hypate, parhypate, lichanos (or hypermese), 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 'unmovable', fixed 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 (chroai) 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.