A metal push switch must have at least two terminals, one for the current to (potentially) go in, another to (potentially) come out. That only describes the simplest version of a switch though. More often than not, a switch has more than two pins. So how do all of those terminals line up with the internal workings of the switch? This is where knowing how many poles and throws a switch has is essential.
The number of poles on a metal button switch defines how many separate circuits the switch can control. So a switch with one pole, can only influence one single circuit. A four-pole switch can separately control four different circuits.
A metal switch’s throw-count defines how many positions each of the switch’s poles can be connected to. For example, if a switch has two throws, each circuit (pole) in the switch can be connected to one of two terminals.
Knowing how many poles and throws astainless steel switch has, it can be more specifically classified. Commonly you’ll see switches defined as “single-pole, single-throw”, “single-pole, double-throw”, “double-pole, double-throw”, which are more often abbreviated down to SPST, SPDT, and DPDT, respectively.
A single-pole, single-throw (SPST) switch is as simple as it gets. It’s got one output and one input. The waterproof switch will either be closed or completely disconnected. SPST are perfect for on-off switching. They’re also a very common form of momentary switches. SPST vandal resistant switches should only require two terminals.
Another common switch-type is the SPDT. SPDTs have three terminals: one common pin and two pins which vie for connection to the common. SPDTs are great for selecting between two power sources, swapping inputs, or whatever it is you do with two circuits trying to go one place. Most metal push button switches are of the SPDT variety. SPDT switches should usually have three terminals or 5termianls including 2poles for led lighting. (Sidenote: in a pinch an SPDT can actually be made into an SPST by just leaving one of the switch throws unconnected).
Adding another pole to the SPDT creates a double-pole, double-throw (DPDT) switch. Basically two SPDT vandal proof metal switches, which can control two separate circuits, but are always switched together by a single actuator. DPDTs should have six terminals plus two terminals for led .