If a top is spinning in the horizontal x,y planes in a counter-clockwise motion, the vector of its angular velocity and acceleration is perpendicular to that plane, specifically in the positive z direction. Is there any specific reason for direction? My book states that the vector must be perpendicular to the plane of motion so that is unique regardless of whether you observe the object from its top or bottom, but this answer does not seem very satisfactory. Is there another reason? Or is it really arbitrarily based on the fact that when our thumbs curl in a counter-clockwise motion our thumb points upward?