The mixture of rigour and freedom with which Haydn approached the task of writing six new string quartets in 1787 can easily be obscured by any overview of the set. By their very nature, summary descriptions are liable to emphasize the consistencies rather than the variations in a composer's approach; in terms of its critical reception, Op. 50 has thus tended to be marginalized under the heading of ‘monothematicism’. This, as has been pointed out, is a natural enough tendency; the variety, on the other hand, can only be appreciated by investigating each work and movement on its own terms. In the case of the first movements this imbalance is easily remedied. The fact that they all employ what has come to be known as sonata form is of far less interest than the diversity of material and technique that they employ, as Somfai and Rosen have affirmed; and the following approaches to Op. 50 will be less concerned with this formal aspect of the first movements than with the model they provide for the musical action of the entire work. Indeed, sonata form itself in the Classical period must be understood not as a set of specific structural demands, as in the old textbook definitions, but as a principle that may apply to any external design. This sonata principle involves a sensitivity to the balance of stable and unstable elements – not just harmonic, but also motivic and textural – in which the instability is concentrated towards the middle of the formal arch and the stability is established particularly towards the end of the structure.