Relays come in various configurations for their switch contacts. They may be as simple as an on/off switch, or as complex as integrating several switches into one unit. In a "double pole" configuration, one switch terminal toggles between two different output terminals. Regardless of the configuration, each switch on a relay can be "normally open" (NO) or "normally closed" (NC), that is, when the coil is at rest and not energised (no current flowing through it), the switch contacts are given the designation of being NO or NC. In an open circuit, no current flows, such as a wall light switch in your home in a position that the light is off. In a closed circuit, metal switch contacts touch each other to complete a circuit, and current flows, similar to turning a light switch to the "on" position. In the accompanying schematic diagram, points A and B connect to the coil. Points C and D connect to the switch. When a voltage is applied across the coil at points A and B, an electromagnetic field is created which attracts a lever in the switch, causing it to make or break contact in the circuit at points C and D (depending if the design is NO or NC). The switch contacts remain in this state until the voltage to the coil is removed. A spring pulls the metal contact back, breaking the connection.