Works titled "Piano Trio" tend to be in the same overall shape as a sonata. Initially this was in the three movement form, though some of Haydn's have two movements. Mozart, in five late works, is generally credited with transforming the accompanied keyboard sonata, in which the essentially optional cello doubles the bass of the keyboard left hand, into the balanced trio which has since been a central form of chamber music. With the early 19th century, particularly Beethoven, this genre was felt to be more appropriate to cast in the four movement form. Piano trios that are set in the Sonata tradition share the general concerns of such works for their era, and often are reflective directly of symphonic practice with individual movements laid out according to the composer's understanding of the sonata form.