The most commonly used relay in car audio and security is the Bosch type relay (AKA: 5-pin relay, AKA: 5-prong relay, AKA: 5-terminal relay...). Although Bosch is no longer producing the relay (Tyco purchased the relay division from Bosch), it's still referred to as a Bosch relay. On this page, it will also be referred to as a Bosch type relay. The photo below is the bottom of the relay. Take note of the markings (85, 86, 87, 87a, & 30) near the terminals. 85 and 86 are the relay coil terminals. 30, 87 and 87a are the various contacts. This is a double-throw relay and has five terminals (two for the coil and three for the contacts). A single-throw relay would have only four terminals (two for the coil and two for the contacts).