When a man sits down at a desk to work on a computation, he has six things on his desk to work with: a work sheet; a desk calculator, to add, subtract, multiply, and divide; some rules to be followed; the tables of numbers he will need; the data for the problem; and an answer sheet. In his head, he has the capacity to make decisions and to do his work in a certain sequence of steps. These seven subdivisions of calculation are all found in the Bell Laboratories’ general-purpose relay computer. The general-purpose computer is a computing system, in fact, more than it is a single machine. The part of the system which does the actual calculating is called, in the following paragraphs, the _computer_, or else, since it is in two halves, _Computer 1_ and _Computer 2_. Physical Units The computing system delivered to the Ballistic Research Laboratories fills a room about 30 by 40 feet and consists of the following: 2 _computers_: panels of relays, wiring, etc., which add, subtract, multiply, divide, select, decide, control, etc. 4 _problem positions_: tables each holding 12 mechanisms for feeding paper tape, which read numbers and instructions punched on tape and convert them into electrical impulses. 2 _hand perforators_: keyboard devices for punching instructions and numbers on paper tape. 1 _processor_: a table holding mechanisms for feeding 2 paper tapes and punching a third paper tape, used for checking numbers and instructions punched on tape. 2 _recorders_: each a table holding a teletypewriter, a tape punch, and a tape feed, used for recording answers and, if necessary, consulting them again. The 2 computers correspond to the work sheet, the desk calculator, and the man’s capacity to make decisions and to carry out a sequence of steps. The 4 problem positions correspond to the problem data, the rules, and the tables of numbers. The 2 recorders correspond to the answer sheet. The 2 hand perforators and the processor are auxiliary machines: they translate the ordinary language of arithmetic into the machine language of punched holes in paper tape. This is the computing system as organized for the Ballistic Research Laboratories at Aberdeen. The one for the National Advisory Committee for Aeronautics has only 3 problem positions. The computer system may, in fact, be organized with 1 to 10 computers and with 1 to 20 problem positions. The great bulk of this computing system, like the mechanical brains described in previous chapters, is made up of large numbers of identical parts of only a few kinds. These are: standard telephone relays; wire; and standard _teletype transmitters_, mechanisms that read punched paper tape and produce electrical impulses. Numbers