start bit Ancient modems alternated between two tones. One tone represented 0 and the other represented 1, which covered the entire binary language. Silence was not an option: to a modem, silence meant the phone call had been cut off. So when a modem had nothing to say, it continuously transmitted the 1 tone to tell the other modem that it was in an idle state. When there was more data to send, the modem would send a single 0 bit to say there was real data on the way. The next eight bits the receiving modem heard were the real data. Then the line went back to idle for at least one more bit. The 0 just before the data is called the start bit. The 1 at the end is the stop bit. See also: asynchronous communication