Why does it require four vectors to calculate a torsion angle?

Question

The BioPython documentation shows the following:

11.6.3 Measuring torsion angles Use the vector representation of the atomic coordinates and the calc dihedral function from the Vector module:

>>> vector1 = atom1.get_vector()
>>> vector2 = atom2.get_vector()
>>> vector3 = atom3.get_vector()
>>> vector4 = atom4.get_vector()
>>> angle = calc_dihedral(vector1, vector2, vector3, vector4)

We know that a torsion angle between N-Cα and Cα-C planes.

So, the minimum number of points required to form a plane is 3.

Then, Why does BioPython require 4 vectors?