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Normally-open (or NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. This type of relay is also referred to as "Form A" or a "make" contact.
Normally-closed (or NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. This type of relay is also referred to as "Form B" or a "break" contact.
Change-over (or double-throw) contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. This type of relay is also referred to as a "Form C" or "transfer" contact. It is also sometimes called a "break before make" contact. The converse, a "make before break" contact, is what is referred to as "Form D".
In addition to these naming conventions, there are other types of relays that are commonly encountered:
Single Pole Single Throw (SPST) - This type of relay has two terminals that can be connected or disconnected. Including the two terminals for the coil, this type of relays has four terminals in total. It is ambiguous whether the contact is normally-open or normally closed. The terminology "SPNO" and "SPNC" are used to resolve the ambiguity.
Single Pole Double Throw (SPDT) - A common terminal connect to either of two other. Including the two terminals for the coil, this type of relay has five terminals in total.
Double Pole Single Throw (DPST) - This type of relay has two pair of terminals. It is equivalent to two SPST relays actuated by a single coil. Including the two terminals for the could, this type of relay has six terminals total. The poles are also ambiguous; they can be normally-open, normally-closed, or one of each.
Double Pole Double Throw (DPDT) - This type of relay has two rows of change-over terminals. It is equivalent to two SPDT relays actuated by a single coil. Including the two for the coil, this type of relay has eight terminals.
Quadruple Pole Double Throw (QTDP) - These relays can also be referred to as Quad Pole Double Throw or 4PDT. This relay consists of four rows of change-over terminals and is equivalent to four SPDT relays actuated by a single coil or two DPDT relays. In total, there are fourteen terminals that comprise the coil.