Where is the inclinometer "ball" in a knife edged turn? Discussion of steep turns has lead to a question about where the inclinometer ball would be 
if the plane rolled to 90 degrees.  Also, would there be any difference if the plane simply rolled to 90 degrees, rolled to 90 degrees and maintained altitude in a straight path with rudder, or rolled to 90 degrees and executed a "knife edged turn?

Does the ball become irrelevant at these attitudes?
As it was pointed out in the other thread, the ball just rolls around inside a curved glass tube.  It doesn't know what the angle of bank is, or anything other than what direction gravity, and/or centrifugal force is moving it.  Therefore:

At 90 deg AOB in coordinated flight at positive G, the ball would be centered.  The aircraft in this case would be in a descending spiral.

At 90 deg AOB in a "knife edge" straight pass where altitude is maintained with rudder, gravity will pull the ball towards the earth.  Same thing if you were able to make a knife edge turn holding altitude with rudder.