(a.) Having three sides or faces; thus, a trihedral angle is a solid angle bounded by three plane angles.

Janus-like ambidextrous bifacial bifold biform bifurcated binary binate biparous bipartisan bipartite bivalent conduplicate dichotomous dihedral disomatous double double-faced duadic dual dualistic duple duplex duplicate duplicated dyadic flanked geminate geminated handed identical lateral many-sided matched multilateral one-sided polyhedral quadrilateral second secondary sided tetrahedral three-sided trihedral trilateral triquetrous twain twin twinned two two-faced two-level two-ply two-sided two-story twofold unilateral