01 What is the CCITT-1 Code?

https://en.wikipedia.org/wiki/Baudot_code

The original Baudot code (later International Telegraph Alphabet No. 1 (ITA1), CCITT-1) was designed by Émile Baudot in 1870 for a telegraph apparatus he developed.

The code was entered directly from a five-key keyboard; pressing or not pressing a key corresponded to setting or not setting a bit in the 5-bit word (the character code) to be transmitted.

Since with five keys, each of which could be either pressed or not pressed, there are only 32 (= 25) different key combinations (31 if you don’t count the idle position of the keyboard), it would not even have been possible to encode all 26 letters plus 10 digits if Baudot had not introduced a switch code that allowed double assignment of almost all combinations: it defined two spaces. When one was sent, the following characters should be interpreted according to a table of letters; after the other, a table of digits and characters should be used.