Alternatively called software flow control, XON/XOFF or XON and XOFF are control characters used in data transmission. XOFF is used by a receiving device as a way to let the transmitting device know it needs to stop transmitting. When the device is ready to recommence, the XON character is sent. With most environments, XOFF is achieved by pressing Ctrl+S and XON is achieved by pressing Ctrl+Q. Some Sun, Linux, and Unix computers have a "Stop" key on the keyboard stops and starts text on the screen.