Pipe length

The organ builders have always designated the pitch of their organ stops in a length in feet, in relation to the deepest and thus longest pipe of a register.

The reason for this was twofold: on the one hand, the organ builder needed this information as a basis for calculating the other components of the organ and thus ultimately for the space that was required for his organ. On the other hand, this also resulted in an indication of the pitches of the registers to each other and thus the tonal structure. Characteristic was the indication of a pipe of eight feet length (8') for the tone C, in most countries the lowest key of the keyboard.

If the pipe was twice as long for this tone (16'), it sounded one octave lower than written, half as long (4') an octave higher, one third as long (⁸/₃' = 2 ²/₃'), one octave+fifth = one duodecime higher.

An exception were organs in Spain and (many in) Italy: The lowest key of their keyboards was usually not C, but F', so the pipe (a fifth lower in comparison) was 12' long, the relative octaves, fifths etc. were calculated from 12'.