There are two ______ in a ladder diagram drawn as vertical lines at the far ends of the page. In a relay logic circuit they would represent the active and zero volt connections of the power supply.

• A. Inputs
• B. Outputs
• C. Coils
• D. Rails

Answer: (D)

In ladder diagrams, the two vertical lines drawn at the far ends of the page are rails. They carry the power from the source: one line is the active (hot) connection and the other is the zero volt (return) connection. The horizontal rungs form the actual control paths between these rails, with devices like coils and contacts placed between them. Knowing that the rails are the power supply paths helps you see how the circuit gets energized when a rung provides a complete path from hot to neutral. The other elements—inputs, outputs, and coils—are real components placed within the rungs, whereas rails are simply the supply rails.