In solid geometry it is used only in a proposition relating to the volumes of two triangular pyramids having a common Trihedral angle, and this is usually omitted.
"The Teaching of Geometry"
David Eugene Smith
Conceive of the figure as in space, so that M is the vertex of a Trihedral angle of which the given triangles are plane sections.
"An Elementary Course in Synthetic Projective Geometry"
Lehmer, Derrick Norman