Short octave
A space-saving key layout in the lowest octave of early keyboard instruments.
What it means
The short octave is a keyboard layout used in the lowest octave of organs, harpsichords and clavichords up to the 18th century, in which rarely needed chromatic pitches were left out and lower diatonic pitches placed there instead, shortening the keyboard.
In the notation
Visually the lowest octave of a short-octave keyboard looks like a normal octave with the usual black and white keys, but sounds different from what the key labels suggest: the lowest key, labelled as an E, actually sounds as a C, so that C sharp and D sharp, rarely needed in this low range, are simply omitted.
Musical use
The advantage of the short octave was that a wider range could fit on a shorter, cheaper keyboard with fewer pipes or strings in the lowest range, since the rarely used low semitones were hardly missed anyway. Later instruments increasingly replaced the short octave with the "broken short octave", where additional split black keys partly restored the missing chromatic pitches, before the fully chromatic keyboard eventually prevailed.