A Dictionary of Music and Musicians/Diapason

DIAPASON originally meant the interval of an octave, because it was δια πασων χορδων συμφωνία, the consonance arrived at by going 'through all the strings of the lyre' from first to last. In this sense it is used by Dryden:

'Through all the compass of the notes it ran,
The diapason closing full in man.'

In French it came to mean a tuning-fork, and hence also the pitch which was as it were registered by it, the 'Diapason normal' being the standard of pitch supposed to be generally accepted in France, which gave 435 vibrations for the A above middle C. In England the name is given to the most important foundation stops of the organ. (See Organ.)

[ C. H. H. P. ]