Positions refer to actors on a stage, but could be generalized to other vertebrates in other settings. The core concept is to focus on the actor's centre of gravity. The relationship to the stage is represented by the plumb line of gravity. The body is drawn outward from the centre of gravity. Assuming a standing (supported) position, the height is calculated so that the support (e.g. the feet) is at the stage floor level.
In any position on the stage, standing straight, the actor can rotate.
The actor might lie down on his stomach, head facing the audience and feet away from the audience. If he started standing straight, facing the audience, he will have rotated about his waist, his centre of gravity - about an axis from one hip-joint to the other. If standing straight is an angle of 0, then using the same right-hand rule, his angle will have increased (thumb toward stage-right from the audience).
The actor might lie down on his left side, so that both head and feet are the same distance from the audience, but the actor is facing the audience. From standing straight and facing the audience, the actor has rotated about an axis from the belly button to the back of the spine. Again, using the right-hand rule, the angle will have increased (thumb toward back of the stage).