IBM’s vacuum tube computers of the 1950s were built from pluggable modules, each holding eight tubes and the associated components. I recently came across one of these modules so I studied its circuitry. This particular module implements five contact debouncing circuits, used to clean up input from a key or relay. When you press a key, the metal contacts tend to bounce a bit before closing, so you end up with multiple open/closed signals, rather than a nice, clean signal. The signal needs to be “debounced” to remove the extra transitions before being processed by a computer.
This is so far before my time, it basically looks like 19th century machinery to me. The steps between this module and what we have today blow my mind.