Standard Modular System
The Standard Modular System (SMS) was a system of standard transistorized circuit boards developed by IBM in the late 1950s, used in their computers and peripherals until the middle 1970s.
As originally anticipated when SMS was developed a set of a couple hundred standard card types would be all that would be needed, making design, manufacture and servicing simpler. Unfortunately that proved far too optimistic as the number of different SMS card types soon grew to well over 2500.