Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical devices such as motor-generators or vibrators. In the United States, the standard "city" dial tone was a 600 Hz tone that was amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current. In the UK, the standard Post Office dialing tone was 33 Hz; it was generated by a motor-driven ringing machine in most exchanges, and by a vibrating-reed generator in the smaller ones. Some later ringing machines also generated a 50 Hz dial tone.