Wikipedia: Rotary dial

The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send interrupted electrical pulses to a remote site equivilant to the number dialed. The dial is circular, about four inches (10 cm) in diameter, with 10 fingerholes cut through the outer perimeter. The dial is mounted via a shaft extending from inside the phone or mounting and sits approximately 1/4 inch (0.7 cm) above a faceplate. The faceplate is set so that through each fingerhole letters and numbers printed on the faceplate may be seen. In the US, traditional dials have letter codes displayed with the numbers under the fingerholes in the following pattern: 1, 2 ABC, 3 DEF, 4 GHI, 5 JKL, 6 MNO, 7 PRS, 8 TUV, 9 WXY and 0 Operator. However, such letter codes were not used in all countries (Australian rotary dial phones never had them). The 1 is normally set at at approximately 60 degrees clockwise from the uppermost point of the dial (if one imagines a clockface over the dial, the 1 is located at approximately the 2 o'clock position), and then the numbers progress upward counterclockwise, with the 0 being at about 5 o'clock. A curved device called a fingerstop sits above the dial at the 4 o'clock position. If a user wants to dial a 6, he or she would put his finger in the 6 fingerhole and rotate the dial clockwise until he reaches the fingerstop. He will then pull out his finger and a spring in the dial will return it slowly to the normal position. As the dial returns, electical contacts wired through the mechanism underneath will open and close 6 times, thus sending 6 pulses to the receiving end.