A 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 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 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 a 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 switch will either be closed or completely disconnected. SPSTs are perfect for on-off switching. They’re also a very common form of momentary switches. SPST switches should only require two terminals.
1× SPST 2 pin Switch