A brief outline of the first movement of one of his concertos will illustrate the manner in which the triplex form was used in all of them, and in which, with few modifications, it has continued to be used by most composers. Let us take the wonderfully delightful concerto in A major (K. 488). The movement opens with a long section for the orchestra. The first theme is announced at once. Later comes the lovely second theme, in the _tonic_ key, be it noted. There is then a short coda, and the orchestra comes to a full tonic cadence and allows the piano to take up the music. The function of this orchestral introduction is to introduce the two themes out of which the movement now proceeds to build itself, conforming pretty closely to the triplex model. The piano has the first theme practically alone, the orchestra merely suggesting an inner voice in the harmony from time to time. In the transitional passage to the dominant key which follows, the piano serves chiefly to spin a few figures over the chords carried by the orchestra. Then the piano has the second theme, now in the dominant, alone; after which it is repeated by the orchestra, the piano adding a touch of ornamental color here and there. Pianoforte and orchestra now play together, the piano taking the rôle of soloist in a series of scales and figures. A full cadence in E major ends the first section. The development section is not long. It will be noticed that the pianist is really soloist through it all, the delicate figure work which he has to perform being always evident above the harmonies or themes of the orchestra. The long opening section for orchestra at the beginning of the concerto is cut down to a few measures in the restatement. The transitional passage between first and second themes is very much shortened likewise. Finally, after the music has progressed duly according to the conventions governing the restatement section in the triplex form, the orchestra makes a pause. Here the pianist is supposed to play what is known as a _cadenza_--a long passage usually testing both him and his instrument to the limit of their abilities. These cadenzas were commonly improvised, and in them Mozart must have displayed the greatness of his power both as a musician and as a player. The cadenza came to an end with a long trill, after which the orchestra, usually without the piano, added the completing coda. The second and third movements were usually in some simpler form. The second was most frequently an aria, the third a rondo. The whole was primarily a piece for the virtuoso, while the orchestra, save when announcing themes or playing ritornelles, served mainly as an accompaniment to the brilliant soloist. It might well, be it understood, carry on the thematic development of the music, thus leaving the pianist free to weave every sort of arabesque; but from now on the concerto was a form of music which was deliberately planned to show off the special qualities of a solo instrument.