Listed only by Irwin, who says: A 2-2/3' manual rank of open metal pipes, composed of mild String tone, perhaps of Viola or soft Salicional quality. It is a Swell, String, or Echo Organ stop. Its train of thirty or so harmonics must not have the peaking in loudness sometimes heard in Strings, in order to be assimilated by the tonal complex. Since each one of these harmonics is not a duplicate of any octave-sounding stop's harmonics, this stop may thicken the tone rather than clarify it, giving mass rather than tonal transparency. A Spitzflöte or conical rank may be more useful than even a mild String. Some examples are String-Flute hybrids. Irwin is incorrect in claiming that a Twelfth stop does not duplicate any harmonics of an octave-sounding stop. Harmonics are integer multiples of the fundamental. If the fundamental is 100 hz, the first 8 harmonics are at 200, 300, 400, 500, 600, 700, 800, and 900 hz. The relative Twelfth would speak with a fundamental of 300. The first two harmonics of this Twelfth are at 600 and 900 hz. Very little math is needed to show that any integer multiple of 300 will also be an integer multiple of 100.
String Twelfth 2-2/3', Great; theater organ, Johannesburg, South Africa; Moeller/Laukhuff 1947. This is the only known example.