Most Greek theorists distinguish three genera of the tetrachord: enharmonic, chromatic, and diatonic. The enharmonic and chromatic genera are defined by the size of their largest incomposite interval (major third and minor third, respectively), which leaves a composite interval of two smaller parts, together referred to as a "pyknon"; in the diatonic genus, no single interval is larger than the other two combined . The earliest theorists to attempt a systematic treatment of octave species, the harmonicists (or school of Eratocles) of the late fifth century BC, confined their attention to the enharmonic genus, with the intervals in the resulting seven octave species being :