In the old days of the Teletype, modems had to idle while the mechanical print
cylinder shifted to the next position ready to type. The modem appended one or two ones
on the end of each character to mark time. Today modems still tack on at least one stop
bit, but for a different reason. The one bit ensures there will be a noticeable
difference (transition from one to zero) when the zero start bit for the next character
is sent. See the essay More
Than You Wanted To Know About RS232C .