 Although in modern usage, a tetrachord may be "any" four-note segment of a scale, or indeed any (unordered) collection of four pitch classes, in ancient Greek music theory a tetrachord consists of a four-note segment of the Greater and Lesser Perfect Systems bounded by the interval of a perfect fourth, the outer notes of which remain fixed in all genera and therefore are called "standing notes" ().The positions of the inner notes vary from one genus to another, for which reason they are called "movable notes" (; from ).In its basic theoretical form, the largest interval of a tetrachord is at the top, and the smallest at the bottom.