The device adds its radius to the radius of convex curves on
the original to produce the convex curves of the copy and it
subtracts its radius from the radius of the concave curves
of the original to produce the concave curves of the copy.

If you 'copy' the new profile(s) recursively on each new copy
the convex curves are broader while the concave curves are
narrower. The resulting profiles approach a straight line.
From then on the device will work fine. For profiles that are
smooth with long radiused curves the device will work 'good
enough' for some range of values of 'good enough'.


for most work of the trim carpentry trades where fitting stuff to
imperfect but pretty good walls, it's plenty good enough. the
introduced error from scribing a 1/2" deep bump 8" or 10" long (like a
bad drywall tape job) is less than the texture on the wall in almost
every case.


the flat washers are almost free, don't take up much space in the kit
and are easy and fast to use.