English: In structural engineering analysis, influence lines are used to depict the variation of quantity at a particular location as the position of a load or action is changed. In the animation shown, we can see the change in bending moment in a statically determinate beam as a unit force moves from one end to the other. Both the bending moment diagram and the influence line for bending moment at the centre of the left-hand span, B, are shown. The influence line is often used as a tool to determine where the load should be placed to cause the most extreme cases of interest, say for design purposes.