= Angle between two planes
{title2=$\theta=\arccos\frac{|\mathbf n_1\cdot\mathbf n_2|}{|\mathbf n_1||\mathbf n_2|}$}
The acute angle between two unoriented planes is the acute angle between their nonzero <normal vectors>, giving the displayed formula. For an oriented convex polyhedron, the <interior dihedral angle of a convex polyhedron> is instead $\pi$ minus the angle between outward face normals. The distinction matters when a face normal is reversed: its unoriented plane is unchanged, but its vector angle is replaced by its supplement.
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