The ceiling should be perpendicular to the wall
(and the wall to the floor).
Even the klutz who built my house got that right.
The right angle for slicing a pizza depends on
how many slices are needed.
Willing to count a circle as a “line” perpendicular to any chosen straight line thru the center? (I am.) If so, then spatial coordinates should almost always (not just usually) be based on perpendicular lines. Want to navigate on a really big pizza? Use polar coordinates.