HVAC Control Ladder Diagram is a diagram used to document industrial control logic systems. They are called "ladder" diagrams because they have two vertical rails and as many "=rungs as there are control circuits to represent. It is a graphical representation of the control logic used in PLC programming. The horizontal control logic lines in the HVAC Control Ladder Diagram are called rungs, and the vertical lines at the start and at the end of each rung are referred to as rails. The name ladder diagram comes from the fact that it resembles a ladder. One of the most usual diagrams used in motor control work is the ladder diagram, also known as a schematic diagram.