I think I am doing a straightforward angle calculation between two points which are -90 degrees apart on a plane, but I am getting -45 degrees from the rs.Angle result.

I’m guessing there is something fundamental about angles in Rhino which I’m not understanding?

Well I’ve found a workaround using rs.Angle2 instead, but it’s too bad I can’t get rs.Angle to work as I’m already generating the plane in order to use it to draw an arc between the two points.

I’m just reading your post without opening your file, I understand you have two points and a plane you created in such a way that the two points both lie upon it. I think that talking about an angle between two points is meaningless. On the other hand, if you have a third point from which you can construct a line to each point you can now find the angle between the two lines at their common (third) point. From your RH view snip it looks like you might be seeking the angle between two lines drawn to each of the points from the grid point where a “horizontal” and “vertical” line (with respect to the grid) intersect. I also don’t have the rs.angle documentation in front of me, but it might be useful to review it, especially for any indication of what it uses if you don’t supply the third point. Maybe it interprets your two points as one line and returns the angle with respect to one of the principal axes?

Hopefully this will give you some thought starters to use while waiting for someone with some solid knowledge to chime in.