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For years, computers didn’t use networks to talk to one another; instead they “dialed up” other computers using a device called a modem which modulated and demodulated signals over a phone line, like whistling at different pitches.
The first modems were very essential with a speaker and microphone, and you socketed your phone’s handset into a special receiver with rubber pads that acted as sound insulation. These acoustic coupler modems were typically capable of transmission speeds of about 150 bits per second, called 150 baud.
This worked well for terminals of the era, like the Teletype Model 33, which could print at about 10 characters per second. Ken Thompson created Unix at Bell Labs using a Teletype 33. Later, Bell Labs upgraded to the Teletype Model 37, which could print in both uppercase and lowercase letters, and included a limited set of mathematical symbols—useful for printing scientific documents. The Teletype Model 37 ran slightly faster at 15 characters per second.
Doing the math: If we assume 1 byte per character and 8 bits per byte, we can convert “characters per second” to “baud” by multiplying by 8, or convert “baud” to “characters per second” by dividing by 8. For example, if the Teletype 37 could print 15 characters per second, that’s about 15 × 8 = 120 baud.
