In music the root (basse fouhuhuhe) of a chord is the note or pitch upon which that chord is perceived or labelled as being built or hierarchically centered upon. This feeling of centeredness is aurally perceivable for those who grew up with European music, and its verbal labelling is a basic skill for the musically trained.
When the root is the bass note, or lowest note, of the expressed chord the chord is said to be in root position. This may also be described as uninverted or in normal form. Often the root is not the lowest pitch played in a chord, in which case the chord is inverted.
Conventionally, the name of the note which is the root is used to denote the chord, thus a major chord built on C is a C Major chord. Starting with Rameau, the analysis and theory of tonal music usually treats the roots as the defining feature of chords and much information can be gained from a progression of roots even if chord inversions are unknown. Also, if the key is known then the chord qualities are known for each root in simple music.
In a root progression, the most familiar form of labelling chord progressions, chords are labelled by their root, rather than bass if different, as above. This is in contrast to an older pre-tonal conception of chords as sonorities wherein root position or first inversion triads are simply considered alternative and fairly equivalent ways of "filling in" the consonance between octaves, C (E G) C or C (F A) C. See: Figured bass.
Although the concept of root is cultural, it does have some basis in the physical properties of waves. When two notes of an interval from the harmonic series are played at the same time, people sometimes perceive the fundamental note of the interval. For example, if notes with frequency ratios of 7:6 (a septimal minor third) were played, people could perceive a note whose frequency was 1/6th of the lower interval. The following sound file demonstrates this phenomenon, using sine waves, pure and simple waves for which this phenomena is most easily evident.
This concept formed the basis for the method by which the composer Paul Hindemith used to determine and identify roots of chords in his harmonic system which he used both to write music and to analyze the music of other composers.