Equilibrium of three parallel forces in one plane

An early day expression used to denote the conditions of equilibrium of three forces in one plane. They are as follows: First, the three parallel forces applied to one body must balance each other and lie in the same plane. Second, the two extreme forces must act in the same direction. Third, the middle force must act in the opposite direction. Fourth, the magnitude of each force must be proportional to the distance between the other two. Same as Laws of the Lever or Lever Rule.