Flow Diagrams, Turing Machines And Languages With Only Two Formation Rules In the first part of the paper, flow diagrams are introduced to represent inter al. mappings of a set into itself. Although not every diagram is decomposable into a finite number of given base diagrams, this becomes true at a semantical level due to a suitable extension of the given set and of the basic mappings defined in it. Two normalization methods of flow diagrams are given. The first has three base diagrams; the second, only two. In the second part of the paper, the second method is applied to the theory of Turing machines. With every Turing machine provided with a two-way half-tape, there is associated a similar machine, doing essentially the same job, but working on a tape obtained from the first one by interspersing alternate blank squares. The new machine belongs to the family, elsewhere introduced, generated by composition and iteration from the two machines L and R. That family is a proper subfamily of the whole family of Turing machines. CACM May, 1966 Bohm, C. Jacopini, G. CA660512 JB March 3, 1978 9:35 AM 1425 4 1425 1781 4 1425 438 4 1425 762 4 1425 249 5 1425 1425 5 1425 1425 5 1425 1425 5 1425 2709 5 1425 2802 5 1425 3004 5 1425 1425 6 1425 1425 6 1425 1425 6 1425 2138 6 1425 2204 6 1425 2247 6 1425 2356 6 1425 2456 6 1425 2456 6 1425 2477 6 1425 3186 6 1425