Imagine a vectorialized typographic "C" which stroke width varies along the curve, as it often happens in typography.
I'd like to draw a curve between those two boundary curves in such a way that each point of the curve is at the same distance ratio from these two curves, for example it is always at the midpoint (distance from outer = distance from inner) or at 33% (distance from outer = twice the distance from inner).
Tracing several different curves with different distance ratios will produce a series of "concentric" curves that gradually change in shape from the outher curve from the inner curve, a little like the lanes of a street seen in a distorted perspective.
(While I used the fixed ratios procedure to let you all understand the desired result, other methods that produce a similar result are fine.)
How can this be done in Illustrator?