In music, the second factor of a chord is the note or pitch two scale degrees above the root or tonal center. When the second is the bass note, or lowest note, of the expressed chord, the chord is in third inversion Play. However, this is equivalent to a gapped eleventh chord.
Conventionally, the second is third in importance to the root, fifth, and third, being an added tone. It is generally not allowed as the root since that inversion resembles an eleventh chord on the second rather than an added tone chord on the original note. In jazz chords and theory, the second is required due to its being an added tone.
The quality of the second may be determined by the scale or may be indicated. For example, in both a major and minor scale a diatonic second added to the tonic chord will be major (C-D-E-G or C-D-E♭-G) while one added to the dominant chord will be major or minor (G-A-B-D or G-A♭-B♭-D), respectively.
The second is octave equivalent to the ninth. If one could cut out the note in between the fifth and the ninth and then drop the ninth down an octave to a second, one would have a second chord (CEGB♭D' – B♭ = CDEG). The difference between sus2 and add9 is conventionally the absence or presence, respectively, of the third.
Read more about this topic: Ninth Chord