Lines do lie in in a plane - well maybe not â€˜aâ€™ plane, rather an infinite number of them - so once you have a plane, one can test to see if a line is in it or not - both endpoints just have to be on the plane.

So, lines, while being theoretically planar curves, should not have a unique plane. However, it seems that in Rhino, if they are parallel to only one of the *principal* planes, they do get assigned that plane. That means that lines parallel to the X, Y and Z axes do not have a plane, but a diagonal line that lies on the world XY plane for example will get assigned that as a plane.

So, to make a script reliable, one maybe has to exclude lines as possible plane source objects - the one you choose first to compare others to - but, a simple test *can* determine if in fact they do lie in a chosen plane afterward.