When Bach played for strangers, he was fond of astonishing them by his originality in registration. "After having first of all censured as ill-advised the combination of certain stops," says Forkel,[188] "the listeners were greatly surprised upon hearing the admirable effect produced by these very combinations, suddenly drawing from the organ a sonority at once original and varied, whose attainment might have been vainly sought by following older methods....
"In trying an unfamiliar organ, his first step was to draw all the registers and to play upon the great manual with all couplers. He was in the habit of saying, jestingly, that he wished at the outset to know if the instrument possessed good lungs."
With this art in registration was combined the greatest facility in improvisation.
"It was often the case," writes Kirnberger,[189] "that friends asked Bach to play to them at times other than during religious service. Then he would choose some theme and treat it in every form of organ composition, playing without interruption for two hours or more, yet without exhausting his resources. Perhaps he made use of his subject first in a prelude and fugue for all the foundation stops. Then his genius in registration was displayed in a movement in three, or in four, parts, always upon the same theme. Now followed a chorale, and the subject served as a counterpoint to the chorale-melody, in ingenious imitations in three or four voices. Finally he concluded by a fugue for organo pleno, based upon the same subject, interweaving the previous variations of it he had made."
IV
In a technical work compiled for his son Friedemann, Bach left us an explanation of the signs employed by him to indicate the various ornaments which he calls Manieren. They are thus illustrated:
Trillo. Mordant. Trillo u. Mordant. Cadence. Doppelcadence.