Support polygon

Stand with your feet apart and imagine a rubber band stretched around the outside edges of both shoes. The area inside is your support polygon.

Lean so that your weight stays inside it and you are fine. Lean past its edge and you have to step or fall. A wide stance gives a big patch; standing on one foot, or on a narrow beam, shrinks it to almost nothing.

This is why robots balance far more easily on two flat feet than on one leg or on a narrow ledge.