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musical scale of four notes, bounded by the interval of a perfect fourth (an interval the size of two and one-half steps, e.g., c-f). In ancient Greek music the descending tetrachord was the basic unit of analysis, and scale systems (called the Greater Perfect System and the Lesser Perfect System) were formed by joining successive tetrachords. Only the outer notes of each tetrachord were fixed; the position of the inner pitches determined the genus of the tetrachord. The basic form was the diatonic genus (e.g., a-g-f-e); its modifications formed the chromatic (a-f-f-e) and enharmonic (a-f-e+-e, with e+ being a pitch between e and f) genera. The Greek theorist Cleonides (c. 2nd century AD) discusses the tetrachord and its genera.