A Turing machine is a hypothetical device with an infinite memory capacity, first conceived by Alan Turing in 1936. The machine manipulates symbols on a potentially infinite strip of tape according to a table of rules, and can be adapted to simulate the logic of any computer algorithm. While none of the following models have been shown to have more power than the single-tape, one-way infinite, multi-symbol Turing-machine model, their authors defined and used them to investigate questions and solve problems more easily than they could have if they had stayed with Turing's a-machine model.