What are form A form B and form C contacts?

These are contact arrangements that can be found on switches and relays. The form designation is fairly old and rarely used now. Form A: a single, normally open contact that closes upon actuation. Form B: same as A, except in reverse. A normally closed contact that opens. Form C: A form A contact connected to a form B. The Form C contact has three wires, NO (normally open), NC (normally closed) and C (common). Upon actuation, the NO contact closes (continuity from NO-C) and the NC contact opens (no continuity from NC-C). It is more common now to see the following designations: Forms A and B can also be referred to as single pole-single throw: SPST-NO = Form A SPST-NC = Form B and Form C can be referred to as single pole-double throw: SPDT = Form C