SPDT (Single Pole Double Throw) A SPDT switching element has one normally open, one normally closed and one common terminal. Three terminals mean that the switch can be wired with the circuit either normally open (N/O) or normally closed (N/C).
DPDT (Double Pole Double Throw) A DPDT switching element has six electrical terminals. In simple terms, it is two SPDT switching elements operating simultaneously at the same settings. This type of switch can handle two independent circuits without using a relay.
Normally Closed Switching Element A Normally Closed (NC) Switching Element is one in which the terminals are wired so that current can flow through the switching element until the plunger pin is actuated to open the circuit.