  The tape never holds more than a finite number of marked squares at any point in a computation. Thus it can be thought of as being finite, but always capable of further extension as required. Obviously this unbounded extendibility is unphysical, but the definition is still of practical use: it means that anything done on a finite tape, however large, is computable. (Turing himself took such a finitistic approach when explaining the practical relevance of computability in his 1950 paper.) One aspect of Turing's formulation, however, involves absolute finiteness: the table of behaviour of a Turing machine must be finite, since Turing allows only a finite number of ‘configurations’ of a Turing machine, and only a finite repertoire of symbols which can be marked on the tape. This is essentially equivalent to allowing only computer programs with finite lengths of code.