Jacques van Oortmerssen's 'Organ Technique' is a thoroughly researched, informative and well written article that lays out a systematic approach to developing organ/keyboard technique based on a relationship between fingering and the correct phrasing of music according to its style, period and genre.
He gives an overview of the evolution of keyboard technique from the 17th to the 20th century. For my own interest and, potentially, that of others, in this and the blogs that follow, I summarise various sections of his article, beginning with finger substitution:
16th and 17th Centuries - Silent finger substitutions were not used
18th Century - Substitution (Ablosender Finger) only on long notes and probably for reasons of sonority, had been known since Couperin.
*In the second half of the 18th century, it was more frequently used in chorale-playing to produce a strong legato.
C.P.E. objects to the use of substitution - "zu oft und ohne Noth." [a technique used (by Couperin) 'too often and unnecessarily.']
The full discussion from Bach's 'Essay on the True Art of Playing Keyboard Instruments' is as follows:
"In the Lessons, when two numerals appear in succession over a single note, the first of the assigned fingers is not released until the second has arrived, for such notes are to be sounded only once unless an embellishment intervenes. Occasionally it is needed in order to sustain the tones of an arpeggio. The flexibility of the thumb makes it well suited for replacement. Because it is not easy to employ this device skilfully it is correctly restricted to relatively long notes and cases of necessity. This precaution should be heeded in the use of all expedients which, partly by their nature, partly by their unusualness, are and remain difficult. Pupils should not be permitted to employ them except as a last resort, or to avoid an even greater difficulty. Couperin, who is otherwise so sound, calls for replacement too frequently and casually. Undoubtedly, the thumb's correct use was not fully known in his time, as suggested by some of his fingered examples in which he replaces the fingers instead of using the thumb or the repeated finger, both of which are easier. Because our forerunners rarely used the thumb, it got in the way. Hence, they often found that they had too many fingers. Gradually, it began to play a more active role, but traces of the old method survived and many were not enterprising enough always to set the thumb on appropriate tones. Today, despite improvements in the use of the fingers, we find, at times, that we have too few of them." [Essay..., p. 73]
19th Century - Silent finger substitution was only allowed on long notes and over short stretches.
*Again, Oortmerssen notes that the use of substitution was unlimited in chorale-playing of this period.
20th Century - Oortmerssen scathingly observes that "...unstructured finger substitution plays a main role, compensating for the lack of thought-out fingering." (p. 41)