A Little More on Pitch, and Transposition
There is no universally 'correct' pitch for music. If you are building an organ, you can get more pipes in less space, and greatly reduce the cost, if you make the pitch high. (If the organ pitch is too high for the singers, the organist could always transpose down a few notes.) The Tsar's brass band used to play at a high pitch, and when he made a gift to the Austrian emperor of a whole outfit of band instruments, the Austrian emperor's band, naturally enough, started to play at that high pitch.
In 1700 the French were using a pitch about a whole tone lower than our modern A 440 (about A 390), and they designed their harpsichords accordingly. In the three hundred years since then, there has been a tendency to drive the pitch up. When violins began abandoning gut strings for iron, the iron sounded better at greater tension, so the necks of violins were lengthened, and the string players sharpened their pitch.
Having a standard international pitch was a convenience for everybody. Sixty years ago this was decreed to be A 440, but many orchestras and opera companies (in spite of the complaints of the singers) are now using A 445, and some violinists like the brilliance of a still higher pitch.
The trouble is that harpsichords don't like to be tuned much higher than A 410 or A 415. We could redesign the nistruments and use shorter scaling, but then we lose soundboard area, and warmth and fullness of tone.
In the past twenty years or so, harpsichord makers have embraced a compromise - they build their instruments for A 415, but provide transposing keyboards, so if you must play at A 440, you can do so by moving the keyboards to the right, so that the C key stands under the C# jack, etc.
This is the compromise, and it is not without its difficulties. Harpsichords want to be tuned from C, and all the historic temperaments (except Equal temperament) are more easily tuned from C. Violins want to be tuned from A, and none of the historic temperaments work terribly well from this note.
If you tune your harpsichord to a good Baroque Chiaroscuro temperament, and then transpose the keyboards, the Key of C, which was the most consonant Key when you tuned the instrument, will become one of the most dissonant, and you won't want to play it. Only Equal temperament, which is one of the most dissonant of all temperaments, can be transposed without retuning. All transposition does is change the overall pitch level of the instrument - but your temperament has been left behind.
If the harpsichord is playing solo, there is no reason for it not to have its best pitch, which is at least a halftone below A 440. You can leave the keyboards in the center of the instrument. If the harpsichord is being used with singers, there is 110 good reason to transpose its pitch. In early music ensembles, where the violins are strung in grit, the lower A 415 pitch is necessary.
It is only when the harpsichord is used with modern instruments that pitch becomes a problem. Eighteenth-century ensembles might take their pitch from the harpsichord, but modern ensembles take their A from a modern oboe, or from an electronic tuning machine.
Violinists with metal strings hate the slacker tension of a lower pitch, and it requires uncommon skill to transpose down a semi tone at sight. Players of keyed wind instruments will have similar problems, unless they have instruments built to sound at the lower pitch.
If your harpsichord must be at A 440 pitch to play with modern instruments, transpose the keyboards, and, if you must, tune in Equal temperament. Your keyboards will always be lopsided to the right, and you will not be able to use the tuning pin pattern to tell you where to put your tuning hammer. It will be difficult to use a historic temperament based on C without upsetting the A for the violinists, so the harpsichord will not sound as well as it might. But you could become familiar with some of the old temperaments and devise your own methods to set them from the A, keeping everyone happy.
David Jacques Way