Such was the sonata form of movement which we have chosen to call the triplex form; a movement in three clear sections, made up of two themes appearing variously in each of them. The three sections are generally known in English as the exposition, the development, and the recapitulation or restatement; and what distinguishes them is the conventional figure or passage work which was used to mark them off, one from the other, and to stand as dividing line between the first and second themes. In the sonatas of Christian Bach all these things are clear and _en règle_; in Emanuel Bach they are obscure. They are clear in the works of the Mannheim group, and in those of the Viennese and Parisian composers who responded to their influence. They are clear in the sonatas and symphonies of Haydn and Mozart and can still be traced in most of those of Beethoven. Hence it would seem that in many ways Emanuel Bach, instead of being the source of the pianoforte sonata, stands very nearly outside the current of influences to which it really owes its most distinctive feature. We may again define the sonata as a piece of music which is a conventional group of several pieces or movements, usually three, more rarely four. The movements are not internally related to each other. The bond which holds them together is only traditional. One of these movements, most often the first, is written in a form sprung of the love of Italians and Slavs for melody, known generally as the _sonata form_. The presence of a movement in this form in a group of pieces will give an unchallenged right to call that group a sonata.[24] III The pianoforte sonata was a sufficiently clearly defined product of musical craftsmanship, if not art, before Haydn and Mozart began seriously to express themselves in it. It is right then to summarize briefly the musical value of the chief sonatas before their day. The many writers may be divided according to the countries in which they practised their art. In London are to be found P. D. Paradies (1710-1792) and Baldassare Galuppi (1706-1785), both Italians, and Johann Christian Bach, submitting almost unconditionally to Italian influence. In the London group too must be reckoned one of the most important men in the development of pianoforte music, Muzio Clementi. In Vienna the chief figure is G. C. Wagenseil; in Paris, Jean Schobert; in Berlin, Emanuel Bach, with whom may be reckoned Friedrich Wilhelm Rust, who, through his brother Johann Ludwig Anton, a pupil of Sebastian Bach, was clearly influenced by the works of the great masters.