 Instead of one tape one can consider a Turing machine with multiple tapes. This turned out the be very useful in several different contexts. For instance, Minsky, used two-tape non-writing Turing machines to prove that a certain decision problem defined by Post (the decision problem for tag systems) is non-Turing computable (Minsky 1961). Hartmanis and Stearns then, in their founding paper for computational complexity theory, proved that any n-tape Turing machine reduces to a single tape Turing machine and so anything that can be computed by an n-tape or multitape Turing machine can also be computed by a single tape Turing machine, and conversely (Hartmanis &amp; Stearns 1965). They used multitape machines because they were considered to be closer to actual digital computers.