Taking the harmonic and voice-leading techniques that his father had developed, he applied them to the homophonic style – allowing dramatic shifts in key and mood, while maintaining an overall coherence. C.P.E. Bach was a decisive influence on Joseph Haydn. One of C.P.E. Bach's most lasting innovations was the shortening of the theme to a motif, which could be shaped more dramatically in pursuit of development. By 1765, C.P.E. Bach's themes, rather than being long melodies, had taken on the style of themes used in sonata form: short, characteristic, and flexible. By linking the changes in the theme to the harmonic function of the section, C.P.E. Bach laid the groundwork that composers such as Haydn and Wolfgang Amadeus Mozart would exploit. The practice of the great Classical masters, specifically Haydn and Mozart, forms the basis for the description of the sonata form. Their works served both as the model for the form, and as the source for new works conceived in the sonata form itself. Debates about sonata form therefore reference the practice of Haydn and Mozart extensively. Joseph Haydn is thought of as "the Father of the Symphony" and "the Father of the String Quartet". He can also be thought of as the father of the sonata form as a means of structuring works. His string quartets and symphonies in particular display not merely the range of applications of the form, but also the way to exploit its dramatic potential. It is predominantly Haydn who created the transition to the development and the transition to the recapitulation, as moments of supreme tension and dramatic interest. It is also Haydn who enabled a more expansive contour for works, by making every aspect of the harmony of a work implicit in its main theme. This is no small innovation, in that it creates a homophonic analog to the polyphonic fugue – a seed of potential from which the composer could later germinate a range of different effects. Haydn's variety of dramatic effects and ability to create tension was remarked upon in his own time: his music was increasingly taken as the standard by which other practice might be judged. Haydn's set of string quartets, Op. 33 gives the first examples of coordinated use of the resources of sonata form in characteristic fashion. The composer himself listed them as being written on completely new principles and marking the turning point in his