In music theory, the bass note of a chord or sonority is the lowest note played or notated. If there are multiple voices it is the note played or notated in the lowest voice. (the note furthest in the bass) While the bass note is often the root or fundamental of the chord, it does not have to be, and sometimes one of the other pitches of the chord will be found in the bass. See: inversion (music).
In pre-tonal theory (Early music), root notes were not considered and thus the bass was the most defining note of a sonority. See: thoroughbass. In pandiatonic chords the bass often does not determine the chord, as is always the case with a nonharmonic bass.