Radio Teletype (RTTY)

To transmit in Morse Code (CW) the transmitter is switched on and off to form a sequence of dots and dashes. In RTTY two different frequencies (called 'mark' and 'space') are transmitted in a timely defined sequence.
The higher frequency is called 'mark' and the lower frequency is called 'space'. The switching between the two frequencies is called FSK (Frequency Shift Keying). The usual frequency shift in Amateur Radio RTTY is 170 Hz, but also shifts of 425 Hz and 850 Hz are used.