In tertiary harmony, such as the major–minor tonal system, the lowest note of a vertical sonority or chord, when the notes of the chord are rearranged as a sequence of 3rds (ex.1a). If no rearrangement is necessary the chord is said to be in root position; if the root is not the lowest note it is said to be in Inversion. Some chords are built in 3rds but lack one or more notes; these can often be interpolated, and the roots can be determined from the completed structure; occasionally this method produces ambiguities (ex.1b). If a chord is based on the superposition of some other interval, for example a 4th, then its root cannot be determined (ex.1c).
JULIAN RUSHTON