The idea of describing an object's orientation in terms of "roll" and "pitch" (and "yaw") are Euler angles (pronounced oil-er). They're the angles of rotation away from an agreed zero, zero, zero orientation. However, when you get near pitch of 90, it gets harder and harder to distinguish what the actual roll is. The best analogy I can think of is to point out that it takes 1000 miles at the equator to go to the next time zone, but it only takes a footstep or two, when you're near Santa's house.