These designations spell out the number of contacts and how they behave during operation. For instance, a basic on/off light switch is an example of a single-pole (i.e., one-circuit) single-throw (i.e., a switch contact in only one position) device that is normally open (NO), or SPSTNO for short. It is also known as a "make" switch. Similarly, a switch that is normally closed (NC) is known as a "break" switch.
The addition of an extra contact to an SPST switch lets it turn off one circuit and turn on another. This makes it a single-pole double-throw (SPDT) switch. Double-throw switches can make connections in either of their two positions. Both circuits share a common signal (pole). SPDT switches can be makebreak, make-before-break, breakmake-break, or break-make-beforebreak or transfer switches.
A double-pole, double-throw (DPDT) switch contains two independent SPDT switches operating from a single actuator. Thus it can control two independent and isolated circuits.
Similarly, switches can have more than two poles. Above two the designation uses a number as, for example, 3PDT. Here a single actuator moves the three sets of contacts simultaneously.
There are special designations for contacts used in electromechanical relays. What are called form designations tell whether contacts open or close when the relay energizes. For example, a Form A relay provides an SPST switch that closes (makes) when a circuit energizes the relay. In contrast, a Form B relay contains a set of contacts that open (break).
Contacts can take on a variety of configurations in relays, with contacts opening and closing in different sequences. Each has its own Form designation. Finally, there are special Form categories for relays that have timedopen or timed-close contacts.