A decision must be made as to which operations shall be built in and which are to be coded into the instructions. Reference has already been made to the uncertain status of division as a built-in operation. Many others, such as forming the logarithm, cosine, arctangent, or square root, have been built into existing machines. Ultimate choice must depend upon the analysis by the designer of the character of the work to be performed by the machine, the frequency of occurrence of operations, and the case with which the non-built-in operations can be compounded from those which are built in. Many operations which are thus excluded from the built-in set are still of sufficiently frequent occurrence to make undesirable the repetition of their coding in detail. For these, magnetic tapes containing the series of orders required for the operation can be prepared once and be made available for use when called for in a particular problem. In order that such subroutines, as they can well be called, be truly general, the machine must be endowed with the ability to modify instructions, such as placing specific quantities into general subroutines. Thus is created a new set of operations which might be said to form a calculus of instructions. Important questions for the users of a machine are: How easily can reference be made to any of the subroutines? How hard is it to initiate a subroutine? What conditions can be used to terminate a subroutine? And with what facility can control revert to any part of the original sequence or some further sequence, as desired? Any machine coding system should be judged quite largely from the point of view of how easy it is for the operator to obtain results. Facilities for conditional and other transfers to subroutines, transfers to still further subroutines, and transfers back again, are certain to be used frequently.