A changeover contact or switch is actually a set of three electrical contacts. There are two fixed contacts with two separate devices, and a contact that moves from one to the other, according to the Relay Company.
On changeover, contact from one device is broken and then established with another. Control of the contacts can be either manual or part of a relay under electronic control. A relay may consist of more than one set of changeover contacts, according to the Relay Company.
A set of three contacts is called a pole. A changeover relay can have one or more poles. The contacts in a pole can consist of materials like tungsten, silver alloys, or gold plating, which are hard and resistant to burnout and corrosion.
Changeover switches can have different configurations. According to the Electronics Club, an SPDT or Single Pole switch is a toggle that just switches from one device to the other. A Double Throw or SPDT Center Off switch has two "On" positions and one "Off" position.
