although it had been believed for some time—and advanced as a theory by —that the sonatas bwv 1014–1019 must have originated in lost trio sonatas for two instruments and continuo, no prior versions have been discovered and it is accepted that only a few movements could have such an origin. the first known source from 1725, in the handwriting of bach's nephew johann heinrich bach, explicitly specifies an obbligato harpsichord; and, despite the fact that a later version in the hand of bach's pupil johann friedrich agricola has a marginal "violin i" at the start of bwv 1014, the scoring of the upper part in the keyboard, especially in the "adagio" movements bwv 1016/i, bwv 1017/iii and bwv 1018/iii, uses figures that are idiomatic to a keyboard instrument but unsuited to other instruments. although this compositional style became widespread in the late eighteenth century, in bach's day it was unusual and innovative. although all the sonatas are written in trio sonata form, each has its own distinct character—the third is an example of the "sonate auf concertenart", a sonata written in the style of a concerto. throughout his life bach returned to the sonatas to refine and perfect the score, particularly in the last sonata, which survives in three different versions.