Air traffic control towers had slanted windows to help ensure focus for the employees.
Large airports are slightly different all over the world, but one constant is the ubiquitous air traffic control tower, which always has windows that slope toward the tower at the base. Many people assume that they are designed that way to prevent the sun's reflection or glare from blinding incoming pilots.
But this explanation doesn't fly, because surrounding buildings (and the airport terminals themselves) have vertical windows.
In fact the benefit is not for those outside the tower but those inside it. Ordinarily, we see (and ignore) reflections in glass all the time, for example from computer monitors or car windows. But air traffic controllers must not have any distracting reflections as they monitor flights. By tilting the glass away, any errant light from inside the tower (such as video screens, lights, etc.) are reflected up onto the ceiling, which is painted black.
That way, the glow from a wristwatch across the room won't be mistaken for an incoming UFO.