SPDT Relay  : (Single Pole Double Throw Relay) an electro-magnetic switch, consist of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one normally open terminal (87).

 

When the coil of the relay is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have continuity. When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity. The diagram to the above right shows the relay at rest (coil not energized). 

Polarity of the coil does not matter. You may attach positive voltage to either terminal of the coil and negative voltage to the other. Diodes are often used across the coil to prevent voltage from feeding back into another circuit.