Tetrachord

In music theory, traditionally, a tetrachord (Greek:τετράχορδoν, Latin:tetrachordum) is a series of four notes ("chords", from the Greek chordon, "string" or "note") separated by 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.