In the following table, Turing's original model allowed only the first three lines that he called N1, N2, N3 (cf. Turing in "The Undecidable", p. 126). He allowed for erasure of the "scanned square" by naming a 0th symbol S0 = "erase" or "blank", etc. However, he did not allow for non-printing, so every instruction-line includes "print symbol Sk" or "erase" (cf. footnote 12 in Post (1947), "The Undecidable", p. 300). The abbreviations are Turing's ("The Undecidable", p. 119). Subsequent to Turing's original paper in 1936–1937, machine-models have allowed all nine possible types of five-tuples: