A Single-Pole Double-Throw (SPDT) or changeover switch has one set ("single throw") of contacts at both switch positions ("double throw").
SPDT can also refer to a relay where it means exactly the same thing except that the contacts are operated by an electromagnet instead of a mechanical lever or button.
Switches can come in various varieties:
- Make-before-break means that when you throw the switch it completes the new circuit before disconnecting the old one. This is not very common, most switches work the other way round.
- Center-off have an additional center position where the switch is completely off.
- Momentary switches are spring-loaded so they return to a default position. When combined with center-off, they can be useful for applications such as operating a twin-coil turnout motor.
For more information see the page on Electrical Devices.