I just came across an interesting tidbit - in WW2 the German Lorenz SZ42 encrypted 50 baud RTTY used three simultaneous tones each for mark and space - providing diversity to overcome fading

Mark 900Hz, 1620Hz, 2340Hz
Space 540Hz, 1260Hz, 1980Hz

At the bottom of this page are some audio files to hear tri-tone and regular RTTY,