This graph shows how Stretch Tune Depth changes relative pitch from low to high notes (left to right). With it OFF, the line is flat: C1 to C2 is exactly an octave, as is C2 to C3, and so on.
At depths 1, 2 and 3, the low octaves are slightly narrower than an octave, the middle (C3 to C4) stays almost exact, and the high octaves (C6 to C7) are slightly wider. The graph isn’t to scale: even at 3 the change is slight.
Stretch tuning is common on keyboard instruments like the piano, where it makes the sound fuller and richer.
Diagram from the XP-80 Owner’s Manual, © 1996 Roland Corporation, reproduced for educational purposes. Credits & disclaimer.