Short octave

The short octave was a method of assigning notes to keys in early keyboard instruments (harpsichord, clavichord, organ), for the purpose of giving the instrument an extended range in the bass. A closely related system, the broken octave, is covered below.

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The short octave

In one variant of the short octave system, the lowest note on the keyboard was nominally E, but the pitch to which it was tuned was actually C. Nominal F# was tuned to D, and nominal G# was tuned to E. Thus, in playing the keys:

E F# G# F G A B C

the player would hear the musical scale of C major in the bass:

C D E F G A B C

The actual note assignments can be seen in the following diagram, which shows the lowest eight keys of an early keyboard.

The rationale behind this system was that the low notes F# and G# are seldom needed in early music. Deep bass notes typically form the root of the chord, and F# and G# chords were seldom used at this time. In contrast, low C and D, both roots of very common chords, are sorely missed if a harpsichord with lowest key E is tuned to match the keyboard layout.

A second type of short octave used the following keys:

B C# D# C D E F# G

to play the G major scale

G A B C D E F# G

Here, the "exotic" bass notes C# and D# are sacrificed to obtain the more essential G and A.

In stringed instruments like the harpsichord, the short octave system created a defect: the strings which were tuned to mismatch their keyboard notes were in general too short to sound the reassigned note with good tone quality. To reach the lower pitch, the strings had to be tuned too slack. During the 17th and 18th centuries, harpsichord builders gradually increased the size and bass range of their instruments, to the point where every bass note could be properly played with its own key.

Short octaves were also sometimes used in the organ. Here, the practice would not have yielded poor tone quality (since the associated pipes would have to be built with the correct length in any event); nevertheless, because of the loss of musical flexibility they entailed, short octaves ultimately came to be abandoned in organs as well.

The broken octave

A variant of the short octave added more notes by using split keys: the front part and the back part of the (visible) key controlled separate levers and hence separate notes. Assume the following keys:

E F F# G G# A

with both F# and G# split front to back. Here, E played C, the front half of the F# key played D, and the (less accessible) rear half played F#. The front half of the G# key played E, and the rear half played F#. As with the short octave, the key labeled E played the lowest note C. Thus, playing the nominal sequence

E F# (front) G# (front) F F# (back) G G# (back) A

the player would hear:

C D E F F# G G# A

The actual note assignments can be seen in the following diagram.

It can be seen that only two notes of the chromatic scale, C# and D#, are missing. An analogous arrangement existed for keyboards with G instead of C at the bottom.

History

According to Trevor Pinnock (reference below), the short octave is characteristic of instruments of the 16th century. He adds, "in the second half of the 17th century, when more accidentals were required in the bass, 'broken octave' was often used."

ources

*"Three Centuries of Harpsichord Making" by Frank Hubbard (1967, Cambridge, MA: Harvard University Press; ISBN 0-674-88845-6)
*Pinnock, Trevor (1975) "Buying a Harpsichord - 1", "Early Music", 126-131.