Soon after the first teleprinters were put into operation, equipment for the encryption of teleprinter signals was constructed. The method first used was simple. Two identically-punched key tapes, one for the sender, the other for the receiver, are produced. They are then glued together in a loop of manageable length, about 1000 characters. The sender punches his plaintext tape and places it in a tape-reader. He then comes to an agreement with the receiver about how the key tapes will be placed in their respective tape-readers, and the transmission starts. In a simple relay circuit a modulo-two addition is performed on the characters from the sender's two tape-readers. A character on a tape can be regarded as a binary number: a combination of `holes' representing ones, and `no holes' representing zeros. A modulo-two addition of two such numbers signifies an addition, without carry, of each bit in corresponding positions. The result of this addition forms the cipher character that is transmitted. The same procedure is used in the receiver. After each transmitted character all tape readers are stepped one position forward and the whole process repeats. The cipher methods gradually evolved. Instead of key tapes a number of code wheels with pins were introduced, e.g. five wheels , one for each channel in the key tape. An active pin had the same function as a hole in the corresponding channel on the tape. Thus it was no longer necessary to punch the plaintext on tape and then transmit it later. The teleprinter could be directly connected to the cipher equipment, hence it was possible to transmit and receive in ``real time'', so saving a lot of time. In the crypto department this encryption method, consisting of adding a key character to a plaintext character, irrespective of how the key character was generated, was called ``overlaying''. This term is used in the following text.