In his early research, Roger Shepard noticed that for harmonically rich tones (i.e. tones with a number of harmonic partials), sequences of tones that are each separated by a major seventh (that is 11 semitones) have an ambiguous character. Though generally perceived as increasing in frequency, such harmonically rich tones were perceived by some as also decreasing in tone. This ambiguity has the principle that while a major 7th is an interval in which the two tones are relatively far apart in terms of height, they are actually close together in terms of chroma.
In exploring this principle, begin by imagining a tone sequence C-B. On the piano keyboard depicted below, find one of the "C" keys. Then play the white key immediately to its left: this is B. If you can't find these tones, just click on the "C->B #1" button below. The first of these tones (follow the bouncing green dot) is a "C" -- specifically C4, or "middle C." The second key is B3.
Now find the "C" key an octave below the one you just observed being played. Click on that "C" (this will
be C3) and then click the "B" above it. This will be the same "B" we just heard (that is, B3). If
you can't find these, click on the C->B #2 (in the second keyboard displayed). Using pure sine tones, these two
intervals -- the major 7th and the minor 2nd -- sound quite different.