The DPDT relay (Double Pole Double Throw) is quite interesting and can be used in various scenarious, including for changing the direction of a motor as you can see in the picture below. It has 2 terminals and 4 connectors and you can look at the DPDT relay as the equivalent of 2 Single Pole Double Throw SPDT relays.
Change motor direction with DPDT relay
As you can see in the schematic the 12V battery (or use other voltages) is connected with the plus at terminal T1 and minus at terminal T2. The contact 1 and 4 are connected together as 2 and 3 are too.
Without voltage applied to the coil the battery plus is connected to contact 1 (and 4) and minus to 3 (and 2) therefore the motor is turning in one direction (let’s say clockwise). When voltage is applied to the coil then the relay switches and now T1 (plus) is connected to contact 2 (and 3) and T2 (minus) is connected to 4 (and 1) therefore the motor is changing the direction of rotation.
Switch between 2 different Loads with DPDT switch
In this second application we can use the DPDT switch to choose between 2 different load and have a visual indication of what load is connected. In our case (figure 1) when there is no voltage applied to the coil the fan and green LED are turned ON . When we apply voltage to the coil (figure 2) the relay switches its contacts and now the light buld and red LED are turned ON.
Related product: Electromechanical Relay