Occupying a special place among Boismortier’s numerous (almost 260) sonatas are the Six sonates à quatre parties différentes et également travaillées Op. 34 (1731), his only surviving quartet sonatas (or sonatas for three instruments and continuo). They contain some of his finest and most accomplished music. Composed in the ‘Italian’ style, they are an extension of his sonata writing for one or two instruments and continuo, and yet their rigid adherence to a four-movement scheme that excludes dances and other movements in binary form (very unusual for Boismortier) suggests that with these works he sought to distance himself from the Corellian model that had inspired his earlier sonatas.
While it is generally held that the Op. 34 sonatas were written for three flutes and bass, they were actually envisaged for violins with organ continuo. As the title indicates, all four instruments are treated equally throughout, most of the writing being based on imitation and the division of melodic material. Boismortier evidently conceived them as larger in scale than his previously composed trio sonatas, not only in terms of their scoring but also regarding their contrapuntal content and the fact that all movements are through-composed. Ironically, the Op. 34 sonatas are relatively short works (none of the movements contains repeated sections), and Sonata II ends with what must be one of the shortest sonata finales ever written. Indeed, the only criticism that might be voiced against these wonderful works is that Boismortier did not make them longer!