= Interior dihedral angle of a convex polyhedron
{title2=$\cos\theta=-\hat{\mathbf n}_1\cdot\hat{\mathbf n}_2$}
At an edge of a convex <polyhedron>, its interior dihedral angle is the angle of the wedge inside the solid, measured in a <plane> perpendicular to the edge. If the two incident faces have outward <unit normal vectors> $\hat{\mathbf n}_1,\hat{\mathbf n}_2$, then their normal angle is supplementary to the interior angle. Thus $\cos\theta=-\hat{\mathbf n}_1\cdot\hat{\mathbf n}_2$. Equivalently, subtract from each vector pointing into one face its <orthogonal projection> onto the common edge; the angle between the resulting inward face directions is the interior angle. This distinguishes an interior angle from the acute angle convention for two unoriented <planes>.
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