 Secondly, one sees that in the 1950s there is a need for theoretical models to reflect on the new computing machines, their abilities and limitations and this in a more systematic manner. It is in that context that the theoretical work already done was picked up. One important development is automata theory in which one can situate, amongst others, the development of other machine models like the register machine model or the Wang B machine model which are, ultimately, rooted in Turing’s and Post’s machines; there are the minimal machine designs discussed in  Section 5.2;  and there is the use of Turing machines in the context of what would become the origins of formal language theory, viz the study of different classes of machines with respect to the different “languages” they can recognize and so also their limitations and strengths. It are these more theoretical developments that contributed to the establishment of  computational complexity theory  in the 1960s. Of course, besides Turing machines, other models also played and play an important role in these developments. Still, within theoretical computer science it is mostly the Turing machine which remains the model, even today. Indeed, when in 1965 one of the founding papers of computational complexity theory (Hartmanis &amp; Stearns 1965) is published, it is the multitape Turing machine which is introduced as the standard model for the computer.