Abstract Subsemitones English
from http://www.hetorgel.nl/e2000-06b.htm
Split keys offered a rather convenient way to exceed the limitations of restricted temperaments and tunings in keyboard instruments, since the essential features of these temperaments were kept, like for example the pure major thirds of meantone temperament.
It seems to have been Italian organ builders of the mid-15th-C. who were the first to apply split keys in organ building. Italy was the main center of this development for the first 150 years. Shortly after 1600 however, Germany, which was dominated by musicians under Italian influence, took the central position.
Split keys were promoted by the Wolfenbüttel ‘Hofkapellmeister’ Michael Praetorius and the Saxon court organ builder Gottfried Fritzsche. From this geographical area the idea spread to surrounding regions and countries. With the rise of circulating temperaments the practice disappeared soon after 1700.
About 70 Instruments with 13-16 keys per octave are known to have been built in Italy, Spain (?), Germany, the Netherlands, England, Sweden and Denmark.
Ibo Ortgies
Split keys on historical organs between 1468 and 1721
Het ORGEL 96 (2000), nr. 6, 20-26 [summary]