Angle between 2 vectors

After the introduction of the plan, which should turn out to be the default and in the upper version, the component seems to have an inversion of the outputs :confused:. Can anyone explain what is happening?
unnamed.gh (9.2 KB)

Invertion might be cause by the direction of Y vector of the plan?

I don’t know your school profile but you must have skipped that part of classical geometry :slight_smile:

And by default if no plan or vector is provided result of Vector Angle must be between 0 and PI.
https://developer.rhino3d.com/api/RhinoCommon/html/Overload_Rhino_Geometry_Vector3d_VectorAngle.htm

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My math is bad :neutral_face:
I somehow poorly understood the current case.

I still can not understand the logic, how is the angle calculated here (briefly)? The default is clear, but with a plan …