What is represented by two vertical lines in a ladder diagram?

• A. The power supply
• B. The input devices
• C. The output devices
• D. The wiring channels

Answer: (A)

Two vertical lines show the power supply rails that provide the voltage for the circuit. They create the source and return path for current, with devices like input contacts and output coils placed between them on each rung. The left rail is typically the positive supply, and the right rail is the return or neutral. When a rung has a complete path from the left rail through active devices to the right rail, the circuit can energize the corresponding output. Because these rails define the voltage source for the whole diagram, they are interpreted as the power supply rather than the inputs, outputs, or wiring channels.