 As is clear from  Sections 1.1  and  1.2,  there is a variety of definitions of the Turing machine. One can use a quintuple or quadruple notation; one can have different types of symbols or just one; one can have a two-way infinite or a one-way infinite tape; etc. Several other less obvious modifications have been considered and used in the past. These modifications can be of two kinds: generalizations or restrictions. These do not result in “stronger” or “weaker” models. Viz. these modified machines compute no more and no less than the Turing computable functions. This adds to the robustness of the Turing machine definition.