Ladder diagrams are sophisticated schematics commonly used to record logic structures for industrial controls. Ladder diagrams are so-called because they resemble a ladder, with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are control circuits. In ladder diagrams, the load device (lamp, relay coil, solenoid coil, etc.) is almost always drawn on the right-hand side of the rung. Although it makes no difference electrically where the relay coil is located within the rung, the ladder's power supply end must be grounded for stable operation. Each system in the relay rack will be represented by a symbol indicating the relationships between such devices on the ladder diagram.