 Where current definitions of Turing machines usually have only one type of symbols (usually just 0 and 1; it was proven by Shannon that any Turing machine can be reduced to a binary Turing machine (Shannon 1956)) Turing, in his original definition of so-called computing machines, used two kinds of symbols: the figures which consist entirely of 0s and 1s and the so-called symbols of the second kind. These are differentiated on the Turing machine tape by using a system of alternating squares of figures and symbols of the second kind. One sequence of alternating squares contains the figures and is called the sequence of F-squares. It contains the sequence computed by the machine; the other is called the sequence of E-squares. The latter are used to mark F-squares and are there to “assist the memory” (Turing 1936–7: 232). The content of the E-squares is liable to change. F-squares however cannot be changed which means that one cannot implement algorithms whereby earlier computed digits need to be changed. Moreover, the machine will never print a symbol on an F-square if the F-square preceding it has not been computed yet. This usage of F and E-squares can be quite useful (see  Sec. 2.3)  but, as was shown by Emil L. Post, it results in a number of complications (see  Sec. 1.2).