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In Group 3, the tones of a pair are separated by a tritone. In the world of Shepard tones, tritones have a particular status: they constitute an exact bifurcation of the octave, as is depicted by the red ovals labelling C and F#:

Here, proximity is neutralized: the distance between C and F# is the same no matter in which direction you move (clockwise or counter-clockwise).  Again, this is corraborated in our hearing: some tritones sound like they are ascending while others sound as though they are descending. In the following, you will hear eight tritone intervals. Some seem to be ascending while others are descending.