Connectors mounted on the bracket allow the connection of external devices to the card. Depending on the form factor of the motherboard and case, around one to seven expansion cards can be added to a computer system. There are also other factors involved in expansion card capacity. For example, some expansion cards need two slots like some NVidia GeForce FX and newer GeForce graphics cards and there is often a space left to aid cooling on some high-end cards.
Some cards are "low-profile" cards, meaning that they take up little physical space. (There is a "low profile PCI card" standard that specifies a much smaller bracket and board area). The group of expansion cards that are used for external connectivity, such as a network, SAN or modem card, are commonly referred to as input/output cards (or I/O cards).
Dimensions for the metal plate on the expansion card is 120mm wide, and 18mm high.
There are three different kinds of PCI cards: Full-Length, Half-Length, and another type called "Low-Profile" or "Slim." Full-length PCI cards are 107mm (height) X 312mm (long). Half-length PCI cards are 106.68mm (height) X 175.26mm (long). Low-profile PCI cards range from 64.41mm X 119.91mm to 64.41mm X 167.64mm.
The PCI bus was introduced in 1991 as replacement for ISA. The standard (now at version 3.0) is found on PC motherboards to this day. Intel introduced the AGP bus in 1997 as a dedicated video acceleration solution. Though termed a bus, AGP supports only a single card at a time. From 2005 PCI-Express has been replacing both PCI and AGP. This standard, approved [by who?] in 2004, implements the logical PCI protocol over a serial communication interface.
After the S-100 bus, this article above mentions only buses used on IBM-compatible/Windows-Intel PCs. Most other computer lines that were not IBM compatible, including those from Tandy, Commodore, Amiga, and Atari, offered their own expansion buses. Even many video game consoles, such as the Sega Genesis, included expansion buses; at least in the case of the Genesis, the expansion bus was proprietary, and in fact the cartridge slots of many cartridge based consoles (not including the Atari 2600) would qualify as expansion buses, as they exposed both read and write capabilities of the system's internal bus. However, the expansion modules attached to these interfaces, though functionally the same as expansion cards, are not technically expansion cards, due to their physical form.
For their 1000 EX and 1000 HX models, Tandy Computer designed the PLUS expansion interface, an adaptation of the XT-bus supporting cards of a smaller form factor. Because it is electrically compatible with the XT bus (a.k.a. 8-bit ISA or XT-ISA,) a passive adapter can be made to connect XT cards to a PLUS expansion connector. Another feature of PLUS cards is that they are stackable. Another bus that offered stackable expansion modules was the "sidecar" bus used by the IBM PCjr. This may have been electrically the same as or similar to the XT bus; it most certainly had some similarities since both essentially exposed the 8088 CPU's address and data buses, with some buffering and latching, the addition of interrupts and DMA provided by Intel add-on chips, and a few system fault detection lines (Power Good, Memory Check, I/O Channel Check.) Again, PCjr sidecars are not technically expansion cards, but expansion modules, with the only difference being that the sidecar is an expansion card enclosed in a plastic box (with holes exposing the connectors.)