We would like to separate the description of the actor's pose from the description of his position on the stage.
We shall use the triangle between the pelvic girdle where it attaches to the backbone, and his right and left hip joints. The corner of the triangle where the pelvic girdle attaches to the backbone is called 'the top'. We can draw a line from the top to the middle of the line between the right and left hip joints. Where these two lines intersect is called 'the bottom'.
The triangle between the pelvic girdle / backbone and the right and left hip joints defines a plane relative to the three dimensions defined by the stage. If the actor is standing, the line between the top and the bottom of the triangle will intersect the floor of the stage. The lines between the top and the left and the right hip joints can also be extended to intersect with the stage. (in general, these three intersection points with the stage will be in a line.)
A line can be drawn between the top of the triangle and the stage floor, such that there are right angles between the two dimensions of the plane corresponding to the stage floor and the line. Since the actor's centre of gravity is fairly close to the actor's triangle top, this line can be seen as a plumb line that indicates whether the actor is in a stable position.
Assuming both feet are on the floor and carrying weight, we can draw a quadri-lateral (four-sided) figure using toes and heels as the corner positions. If the actor stands fairly straight (does not shift his centre of gravity too far), and if the plumb line intersects the stage floor within this rectangle, the stance or pose of the actor is stable and can be maintained.