How did the "motherboard" get its name? I know that the name fits because it's the main circuit board in the computer, but how did that name come to be?
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Originally, of course, computers were racks and racks of equipment. Printed circuit boards, generally about 5x8 inches, plugged into a "backplane" which had no active components on it. (In fact, the "backplane" was often just wire-wrap sockets with wires connecting the pins -- no printed circuit board.)
Slowly this layout changed. The "S100 bus", the original PC, generally used printed circuits for the backplane. The Apple II was probably the first commercial product that resembled the current "motherboard" with the processor on the main board and multiple expansion slots, though the layout of the board was a bit haphazard. The early IBMPC had a slightly more organized layout, but basically the same. More recently motherboards have diverged from the original Apple II/IBMPC style, incorporating more componentry on the mainboard, with more specialized slots.
The term, as vcsjones suggests, was probably a sort of back-formation from the fact that the plug-in boards would have been called "daughter" boards.