Small form factor is a term used for desktop computers and for some of its components, chassis and motherboard, to indicate that they are designed in accordance with one of several standardized computer form factors intended to minimize the volume and footprint of a desktop computer compared to the standard ATX form factor.
Motherboard form factor comparison (pen for scale)
Shoebox case (Shuttle SP35 Pro)
Acer Aspire Revo
An Intel NUC
Expansion card
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In computing, an expansion card is a printed circuit board that can be inserted into an electrical connector, or expansion slot on a computer's motherboard to add functionality to a computer system. Sometimes the design of the computer's case and motherboard involves placing most of these slots onto a separate, removable card. Typically such cards are referred to as a riser card in part because they project upward from the board and allow expansion cards to be placed above and parallel to the motherboard.
Example of a PCI digital I/O expansion card using a large square chip from PLX Technology to handle the PCI bus interface
Altair 8800b from March 1976 with an 18-slot S-100 backplane which housed both the Intel 8080 mainboard and many expansion boards
Rack of IBM Standard Modular System expansion cards in an IBM 1401 computer using a 16-pin gold plated edge connector first introduced in 1959
Configuration DIP switches in a 16-pin through-hole package as often found in ISA expansion cards from the 1980s