To facilitate an understanding of the subsequent explanations we will give a short technical review of teleprinters.
The first teleprinters were constructed at the end of the 19th century. The principles have remained largely unchanged since then. Every character transmitted consists of a combination of pulses of two types. The number of pulses in a character is always five, contrary to the varying number of pulses in the Morse code alphabet. All pulses are of the same length, and are indicated by positive or negative polarity or alternatively, current or no current. Five pulses taking on two different states give 32 different combinations, but that is not sufficient for all the characters that have to be transmitted. There therefore exists an arrangement with the same function as the letter/figure shift key on a typewriter. A combination of pulses causes all subsequent characters to be received as number or punctuation characters, and another combination signals in a similar way that all the following characters will be received as letters. Several teleprinter alphabets have existed. The one mostly used is called the Murray code after its inventor, or the International Teleprinter Code (International Telegraph Alphabet No. 2). The text is punched on paper tape which is fed into the transmitter. `Hole' or `no hole' in the tape corresponds to `current' or `no current', or to `positive polarity' or `negative polarity'.