In design and treatment it is essentially similar to the earlier triple concerto. There is no intention apparent of making the two instruments independent; the players emulate each other in the delivery of the melodies and passages, sometimes together, sometimes in succession, often breaking off in rapid changes and interruptions; the melodies are sometimes simply repeated, sometimes with variations so divided between the two instruments that neither can be said to have the advantage over the other. There are somewhat greater difficulties of execution than have been usual hitherto, a few passages, for instance, in octaves and thirds, but very modest ones; the passages generally have more variety and elegance. The orchestra is simply and judiciously, but very delicately treated, the wind instruments in sustained chords, as a foundation for the clavier passages; the effect of the _crescendo_ and a greater attention to light and shade show the influence of Mannheim. Altogether the concerto is a well-arranged composition, clear and melodious, as well as accurately constructed, with a free, cheerful expression, which is most strikingly shown in the fresh gaiety of the last movement. As organist, Mozart was under the necessity of playing the organ at festivals, but as a rule only for accompaniments and for interludes at set places, which gave him opportunities for improvising--his special delight. We have some organ sonatas with orchestral accompaniments belonging to this time (328, 329, 336, K.), quite in the style of those already noticed (Vol. I., p. 286); compositions after the fashion of the first movement of a sonata, without a trace of ecclesiastical severity, either in the technical construction, which is very light, or in the style, which is brilliant and cheerful. The organ occurs as an obbligato instrument only in one of these sonatas (329 K.), which is the most elaborated, but still very moderate in style, and without any florid passages. {MASSES, 1779, 1780.} (93) Of more important church compositions there belong to this period two Masses in C major, of which the earlier (317 K.) is one of Mozart's best-known works of the kind, bearing date March 23, 1779, and the later (337 K.) was written in March, 1780.[6] They are quite after the prescribed manner, not too long, not too serious, and yet not light; in no respect difficult or important, and closely allied in substance and treatment to the earlier works which have already been analysed (Vol. I., pp. 263 et seq.). The easy invention, never at a loss for fitting expression, the talent for organisation which arranges the parts into a connected and coherent whole, the technical sureness which gives to every detail its due share of interest--above all, the inexhaustible gift of melody and symmetry: all these qualities are here to be found, and it is by their aid that, in spite of hampering circumstances, such great and healthy work was done.