Pohlke's Theorem

The principal theorem of Axonometry. It states that three segments of arbitrary length $a'x'$, $a'y'$, and $a'z'$ which are drawn in a Plane from a point $a'$ under arbitrary Angles form a parallel projection of three equal segments $ax$, $ay$, and $az$ from the Origin of three Perpendicular coordinate axes. However, only one of the segments or one of the Angles may vanish.

See also Axonometry