When did perpendicular lose its verticality? I have always understood perpendicular as being “at right angles to the plane of the horizon” ie: at right angles and vertical.
OED:- 1. perpendicular, adj. and n. ...Situated or directed at right angles to the plane of the horizon; vertical....
The wall is at perpendicular to the floor but the floor is at right angles to the wall.
But more and more I hear it being used as meaning at right angles regardless of the plane. I have even seen such a reference in print. Once again our good friend Jeffrey Deaver:- “I took a chair perpendicular to his.” Another example of evolution?