Relay sockets are devices that accept relays, electromechanical switches in which the variation of current in one electric circuit controls the flow of electricity in another circuit. Relay sockets are made of metal or composite materials and available in a variety of shapes, sizes, and colors. Performance specifications include number of poles, maximum voltage, and maximum current. Single-pole (SP) sockets are used with relays that control one line with each throw. Double-pole (DP) devices are used with relays that control two lines. Typically, the first line is hot and the second line is neutral. Triple-pole (TP) sockets accept three pairs of contacts. Four-pole devices (4P) receive four pairs of contacts. Additional considerations when selecting relay sockets include electrical rating, dielectric strength, insulation resistance, and temperature range. Some products are flame retardant, shatterproof, or designed for demanding applications. Others include electrical or mechanical on/off indicators.