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In ancient Greek music theory, a genus is a family of divisions of the tetrachord (four notes spanning a perfect fourth) used to create musical scales. The three genera are distinguished by their characteristic largest intervals, between the upper two notes. The diatonic genus has a characteristic interval of a major second, the chromatic genus has a minor third, and the enharmonic genus has a major third. (Note that this original Greek usage of diatonic ("stretched out"), chromatic ("colorful"), and enharmonic ("harmonious") does not generally correspond to the modern definitions of these terms.) The term pyknon ("compression") refers to the remainder of the tetrachord in the chromatic and enharmonic genera, where two adjacent intervals are a semitone or smaller.