TETRACHORD is a four-note contiguous scale segment.
It helps to understand the structural relationship of scales by observing the layout of the circular segments as described in
the cycle (circle) of fifths diagram. As you travel counter-clockwise around the circle the key centers move an interval of a
perfect fourth upward, or a perfect fifth downward using clock-wise motion.
The nearest segment to any key center conforms to a natural phenomenon based on the principle of close relations of the
overtone series; for ex., a scale built on F spans an octave and shares its lower 4-notes (tetrachord) with the upper four
notes (tetrachord) key center of B flat, the nearest segment counter-clockwise.
The Circle of fifths diagram shown at left represents the continuous movement of tonic centers that conforms to
the physical vibrations of sound as sharps are added in the movement around the circle; This is perceived in a
clockwise direction of flow.
The innermost circle displays the relative Minor key signature that is associated with the particular tonic key in the slice.
It is crucial to an understanding of music theory that this chart be committed to memory.