llustration: Figure 11.--ACTION OF HARPSICHORD. A, bottom; B, belly
rail; C, soundboard; D, wrest plank; E, key frame; F, rack; G, key; H,
jack; I, jack rail. Scale, 1:2.]

There is some historical evidence that this actually was the case. In
his _Theatrum Instrumentorum_ Michael Praetorius[1] pictures a polygonal
virginal, which appears to be very much like the many Italian examples
that survive today, and a rectangular virginal that seems to be Flemish.
He specifies that both are _so recht Chor-Thon_ (at regular choir
pitch). Praetorius also shows a harpsichord[2] that looks like a
typical Italian instrument except for the presence of a set of strings
tuned an octave above unison pitch, a rare feature on Italian
harpsichords. This harpsichord is described as _so eine Quart tieffer
alss Chor-Thon_ (a fourth lower than choir pitch), clearly indicating
that single manual keyboard instruments a fourth apart in pitch were in
existence. Since no reason is given for the harpsichord being tuned a
fourth lower than the two virginals, we may assume that the author
considered the matter commonplace enough as to demand no further
elaboration and that instruments a fourth apart in pitch were not rare.

Praetorius does not state that the harpsichord in his illustration was
tuned to a low pitch standard, which was actually used for certain
purposes or in particular localities. He discussed the numerous pitches
in use before and during his time, but the only one that he mentioned as
being a fourth below choir pitch he considered obsolete and suitable
only for plainsong. If the harpsichord was not intended to be tuned to
this standard and used for this purpose, it must have been tuned to
choir pitch and treated as a transposing instrument.

Querinus van Blankenburg,[3] writing in 1739, states:

At that time [the beginning of the 17th century], men had so little
experience in transposition that in order to be able to transpose a
piece a fourth downwards they made a special second keyboard in the
harpsichord for this purpose. This seems incredible, but the very
remarkable proof is the fact that the famous Ruckers from the
beginning of the last century for a period of more than thirty
years made harpsichords only in this way.[4]

That the second manual of the two-manual harpsichord originated as a
device for transposition is well known. In an article titled
"Transposing Keyboards on Extant Flemish H