"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." [Footnote 189: _Die wahren Grundsätze zum Gebrauch der Harmonie_ (Berlin, 1773) p. 53. See also Mizler (_Necrolog_, p. 171) and Forkel (p. 22).] 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: [Music: Trillo. Mordant. Trillo u. Mordant. Cadence. Doppelcadence. idem. Doppelcadence und Mordant. idem. Accent steigend. (rising.) Accent fallend. (falling.) Accent u. Mordant. Accent u. Trillo. idem.] The greater number of these ornaments,[190] as we see by the table, do not begin upon the given note. However, if a turn occur at the beginning of a piece, or if it ornament a characteristic interval (as, for example, in the fugue in _F_ minor), the essential note should be struck first; even if such a rendering produce a discord with the other parts. [Footnote 190: _Clavierbüchlein, vor Wilhelm Friedemann Bach angefangen in Cöthen den 22. Januar, anno 1720._] The mordent--it is the _pincé simple_ or the "pluck" of Chambonnières, Couperin, and Le Bègue, who had borrowed it from lute-players[191]--is generally diatonic, although with this exception: if the note which it affects be marked with an accidental in the same measure, the accidental must be observed in executing the mordent. [Footnote 191: See A. Méreaux: _Les Clavecinistes de 1637 à 1790. Tableau synoptique et comparatif de tous les agréments avec leur signes et leur effet_. Heugel, Paris.]