Twelfth
Interval spanning twelve scale steps: an octave plus a fifth.
What it means
A twelfth is an interval spanning twelve diatonic scale steps and can be described as the sum of an octave and a perfect fifth. Between the lowest and the highest note of a twelfth there thus lies a full octave plus five further steps.
Musical use
As a compound interval, the twelfth plays an important role in naming organ stops, such as the “Twelfth” stop, which sounds a rank of pipes a twelfth above the foundation stop, thereby reinforcing the harmonic series. The twelfth also appears in the harmonic series of brass instruments, as one of the early, easily reachable intervals between certain natural harmonics.
The twelfth should be distinguished from the simple fifth, from which it arises through octave extension, and from the ninth, which at nine steps represents a different compound interval made up of an octave and a second. In harmony, the twelfth is occasionally mentioned instead of the fifth when describing voice leading that extends beyond the octave boundary.
Performance practice
The easiest way to identify a twelfth is to first recognise the fifth it contains and then mentally add an octave, rather than counting through all twelve steps individually. When working with organ stops, listen specifically for how the twelfth stop adds the corresponding fifth in the higher octave to the fundamental, making the overall sound richer in overtones.