The shear at any section of a beam equals the algebraic sum of all the external forces on either side of the section; it is considered positive if the segment of the beam on the left of the cross section tends to move up with respect to the segment on the right. The algebraic sum of all the forces on both sides of the section will, of course, equal zero, since the system is in static equilibrium. It is usually simplest to add all the shearing forces to the left of the section being considered; this sum is equal to the shearing force acting at that point. If this process is repeated for different points over the length of the beam, a complete shear diagram may be constructed.