I feel like I understand vectors having worked with them for many years. But I’m a bit baffled by the angle-between-vectors module in GH because of the optional plane thing. See angle-between-vectors2.gh (11.1 KB)

If I remove the plane, then the angle between the vectors is 45 degrees as I expect. Why does the angle become zero when I add the XY plane? Seems as though the angle between vectors is the same.

Angle is zero with XY plane because imagine your vectors are projected to the XY plane, they would be right on top of each other, so there is zero angle difference. (just take a look at your angles from the Top viewport). Using a plane forces 2D projected evaluation of the vectors on that plane.

“Using a plane forces 2D **projected evaluation** of the vectors on that plane.”

That was the answer I think I was looking for. If the help description for that module said that, I would not be here asking what it meant. It only says it provides for a 2D angle. Only the cognoscenti know what that means. Sorry for the sarcasm. I just get frustrated with GH sometimes. I appreciate your help and explanation.