I see - the tangent tool is for circle tangent to 2 curves of a specified radius.

When you move your mouse around, you are specifying a different radius all the time.

But in these cases there is only 1 possible radius that will work, that's why it looks like it isn't working, because if you ask for a circle of radius 1 unit for example, it cannot fit one in there.

But even if you get the right radius, I guess it still might not work because there are an infinite number of tangent circles of that radius that can slide between those 2 curves, I think it might not find it in this situation also.

- Michael