On each unit of Eniac there are plug hubs or sockets (called _program-pulse input terminals_) to which a program trunk may be connected. A program pulse received here can make the unit act in some desired way. On each accumulator of Eniac, we find 12 program-pulse input hubs. Corresponding to each of these hubs, there is a nine-way switch, called a _program-control switch_. The setting of this switch determines what the accumulator will do when the program-pulse input hub belonging to the switch receives a program pulse. For instance, there are switch settings for: receive input on the α line, receive input on the β line, etc.; and transmit output on the add line, etc. There is even a switch setting that instructs the accumulator to do nothing, and this instruction may be both useful and important. Now, in order that Accumulator 11 may transfer a number to Accumulator 13, we need: (1) a digit tray, say B, for the number to travel along; (2) a program trunk line, say G3, to tell Accumulator 11 when to send the number and Accumulator 13 when to receive it; and (3) certain plugging as follows: 1. We plug from program trunk G3 into a program-pulse input hub, say No. 5, of Accumulator 11; 2. We plug from the same program trunk G3 into a program-pulse input hub, say No. 7, of Accumulator 13; 3. We set program-control switch No. 5 of Accumulator 11 to “add”; 4. We set program-control switch No. 7 of Accumulator 13 to some input, say γ. 5. We plug from digit tray B into the add output of Accumulator 11. 6. We plug from digit tray B into the γ input of Accumulator 13. Now, when the program pulse comes along line G3, it makes Accumulator 13 transmit additively along digit tray B into Accumulator 13. And that is the result that we wanted. As each mechanism of Eniac finishes what it is instructed to do, it may or may not put out a program pulse. This pulse in turn may be plugged into any other program trunk line and may stimulate another mechanism to act. Then, when this mechanism finishes, it too may or may not put out a program pulse, and so on.