Relevant part: As early output devices (Teleprinters, CRTs and line printers) could not produce more than one character at a location, it was not possible to u

Relevant part: As early output devices (Teleprinters, CRTs and line printers) could not produce more than one character at a location, it was not possible to underscore text, so early encodings such as ITA2 and the first versions of ASCII had no underscore. IBM's EBCDIC character-coding system, introduced in 1964, added the underscore, which IBM referred to as the "break character". ASCII added it in 1967.