In music, the fifth tone of the diatonic scale, or a chord based on the fifth tone of the scale as its root. In the C Major scale, G is the fifth, or dominant, tone. A simple chord, or triad, based on G includes the notes G, B and D.
The dominant tone and chord have special qualities and relationships to the tonic in Western music, both classical and popular, from the 17th century to the present. But long before that Pythagoras “discovered” this tonal relationship in his experiments with vibrating strings. He found that stopping a string at a point exactly 2/3 along its length produced this tone. Later named a Perfect Fifth due to this mathematical accuracy, the dominant tone holds the strongest relation to the tonic, or keynote (C in the key of C). This is in large part due to its prominence in the harmonic series, in which the fifth is the first independent tone after the octave to appear as a harmonic, or partial. In fact, in a chart of the harmonics that make up the tone “C,” G (in succeeding octaves) comprises the third, sixth, twelfth and twenty-fourth harmonics of the note.
Chords based on the dominant have a strong tendency to move to the tonic chord for a couple of reasons. First, the dominant chord G-B-D shares a tone with the tonic chord C-E-G. Second, the dominant chord contains the leading tone B, which to our Western ears demands to resolve up to the tonic C, a half step away.