Pushbutton switches are mechanical switches defined by the method used to activate the switch. The activation method is typically in the form of a plunger that is pushed down to open or close the switch.  Pole and throw configurations for pushbutton switches can be single pole single throw (SPST), single pole double throw (SPDT), double pole single throw (DPST), double pole double throw (DPDT), or solid-state.  SPST is a switch that makes or breaks the connection of a single conductor in a single branch circuit. This switch typically has two terminals. It is commonly referred to as a "Single-Pole" Switch.  SPDT is a switch that makes or breaks the connection of a single conductor with either of two other single conductors. This switch typically has 3 terminals, and is commonly used in pairs and called a "Three-Way" switch.  DPST is a switch that makes or breaks the connection of two circuit conductors in a single branch circuit. This switch typically has four terminals.  DPDT is a switch that makes or breaks the connection of two conductors to two separate circuits. This switch typically has six terminals and is available in both momentary and maintained contact versions.  A solid state switching mechanism includes transistors and piezoelectric materials.  A normally open (NO) switch has contacts that are open or disconnected in their unactuated (normal) position.  A normally closed (NC) switch has contacts that are closed or connected in their unactuated (normal) position.