Relay logic is a method of controlling industrial electronic circuits by using relays and contacts. The schematic diagrams for relay logic circuits are often called line diagrams, because the inputs and outputs are essentially drawn in a series of lines. A relay logic circuit is an electrical network consisting of lines, or rungs, in which each line or rung must have continuity to enable the output device. A typical circuit consist of a number of rungs, with each rung controlling an output. This output is controlled by a combination of input and output conditions, such as input switches and control relays. The condition that represent the inputs are connected in series, parallel, or series-parallel to obtain the logic required to drive the output. The relay logic circuit forms an electrical schematic diagram for the control of input and output devices. Relay logic diagrams represent the physical interconnection of devices.

Discrete Mathematics has full theoretical foundations of Relay Logic.