The Tibia Clausa is one of the foundation ranks and arguably the most important rank of pipes in a theatre pipe organ. It provides the basic foundation tone of the organ with few overtones or harmonics. The stop shares similarities with the Bourdon and the Gedeckt found in some church pipe organs. The name Bourdun is derived from the French word for buzz and normally denotes a stopped flute/flue type of pipe in an organ, though in organ building the stop has no buzz, but rather a very dark, heavy tone, strong in fundamental, with little overtone development. ...

The Tibia Clausa, or Tibia, is generally found at 16', 8', 4' and 2' pitches as a unified rank. The mutation ranks Tibia Quint 5-1/3', Nazard 2-2/3' and Tierce 1-3/5' are also drawn from this unified rank of 97 pipes. In some larger organs, a second Tibia rank may be present, extended to 1' instead of 16', allowing a 1-1/3' Nineteenth mutation and a 1' Piccolo to be drawn from this rank. In smaller organs, a Bourdon or Stopped Diapason may be substituted for a Tibia Clausa at 16' pitch. The tierce is an old English unit of wine casks, holding about 159 litres. ... The name Bourdun is derived from the French word for buzz and normally denotes a stopped flute/flue type of pipe in an organ, though in organ building the stop has no buzz, but rather a very dark, heavy tone, strong in fundamental, with little overtone development. ... The word diapason (pronounced ) is another name for the musical interval of the octave, especially in the context of Pythagorean intervals. ...

The Tibia may be voiced on wind pressures from 10" to 25" WP (water pressure). The Tibia is generally used as a chorus stop, with or without tremulant; it is not normally used as a solo stop due to its relatively dull tone.

Other variants of the Tibia Clausa include: Tibia Bass, Tibia Flute, Tibia Major, Tibia Plena and Tibia Rex.