A Harmonic Gedeckt of 2-2/3' speaking length, the longest pipe being 4' actual length. It was first introduced by Thomas Casson at Omagh, County Tyrone, Ireland, and placed at his direction in other organs by the Positive Organ Company of London, including the London Organ School, and Cathcart House, Kensington, S.W. Its tone is full and clear, and blends well, having relatively few upper partials. It is less assertive than the normal Twelfth.

Harmonic Twelfth 2-2/3', Gallery Organ II; Convention Hall, Atlantic City, New Jersey, USA; Midmer-Losh.

Harmonic Stopped Twelfth 2-2/3', Solo; St. Matthew's Lutheran Church, Hanover, Pennsylvania, USA; Austin 1925.