  At the time when Gentzen worked out his system of natural deduction,  Stanislaw Jaskowski was also developing a logical system for  reasoning with assumptions. Formulas in derivations are arranged in a linear succession, but Jaskowski's paper of 1934 remained fragmentary and without substantial results such as a subformula property. The  linear variant of natural deduction is followed in many pedagogical  expositions of elementary logic (sometimes called “Fitch  systems”). Gentzen found Jaskowski's work by June 1936, when  both were in Münster, and considered its linear arrangement of  formulas an improvement, a “liberation from the straitjacket of the tree form”, into one that reflects “the linearity of  thinking” (the former from unpublished notes, the latter from  Gentzen's thesis).