Editing Keyboard terminology (section)

===Switch configurations===
Normal switches are called '''momentary''', as they typically are only closed (connected) for a moment, and will open (disconnect) when you let go. Normal switches will connect a single circuit pathway, or provide only one of some other form of sensing; such a switch is called '''single pole single throw normally open''' (SPST NO). NO is also called '''form A''', and NC is also called '''form B'''.

'''[[Alternate action]]''' switches (also '''latching action''' or '''push-push''') will stay down when pressed; to release them, they must be pressed a second time. Such switches were used for [[lock key]]s in the 70s and early 80s, after which they went out of fashion, with Apple being the last prominent manufacturer to use them, up to the [[Apple Extended Keyboard II]].

'''[[Double action]]''' or '''two-stage''' switches connect two circuit paths in sequence, with the second connection made after the key is pressed further. These are more likely to be found in electric typewriters.

Redundancy can be provided with a second pair of terminals. Here, a single switch with a single plunger connects two separate circuit paths at once; this is called '''double pole single throw''' (DPST). These are not generally found in keyboards, but [[Micro Switch SD Series]] switches are known for having redundant sense lines. In some cases, switches clearly support four terminals, but are seldom if ever seen with the second pair of holes populated, with the available factory configurations left as a mystery. As time passes, and more specifications are acquired, the nature of switches become clear. Some are found to have four terminals for DPST, and some are found to have four terminals for two-stage action.

Some switches have three terminals to provide '''changeover''' action (current is diverted from one circuit to another), or four terminals where one pair of contacts is normally open and the other is normally closed. The latter will function as the former if the input terminals are connected at the PCB. Changeover is also described as '''form C'''.

The increased affordability of microprocessor-driven equipment has removed the necessity of more complex switch configurations for keyboards, as the software takes care of the machine state, but other (non-computer) equipment may also find a use for switches other than SPST NO.