In music, the fourth tone of the diatonic scale, or a chord based on the fourth tone of the scale as its root. In the C Major scale, F is the fourth, or subdominant, tone. A simple chord, or triad, based on F includes the notes F, A and C. The word “subdominant” literally means “below the dominant” tone.
Way back in ancient Greece Pythagoras discovered a special relationship between a tonic note and its Perfect Fourth in his experiments with vibrating strings. He found that stopping a string at a point exactly 3/4 along its length produced this tone. Ancient Greeks found the even integer relationships of this interval, the Perfect Fifth and the Octave to be of profound importance. These days we hear these intervals all the time and aren’t quite so impressed.
Besides its pure relationship to the tonic, the subdominant tone also has a unique interaction with the seventh tone (the Leading Tone) of the scale. This interval of an augmented fourth, also called a tritone, was long perceived to be most unpleasant. In harmony, the subdominant chord also interacts with the tonic in a unique way. It contains the tonic note (again “C” in C Major); in fact, the C chord is the dominant chord for the key of F.