Relay logic is a method of designing and implementing control circuits using electromechanical relays to perform logical operations such as AND, OR, and NOT. Before the advent of digital electronics and programmable logic controllers (PLCs), relay logic was widely used in industrial automation and control systems. Each relay acts as a switch that opens or closes circuits based on electrical signals, enabling complex control sequences. While less common today, understanding relay logic is important for maintaining legacy systems and for grasping the fundamentals of digital logic, as many concepts directly translate to modern electronic logic circuits.