The motherboard must be the same 'form factor' as the case it will be mounted in, which will be either Baby AT or ATX. The IBM PC AT computer introduced the AT form factor, which was later reduced in size to become the Baby AT and is still the most predominant form factor in use today. The ATX form factor was introduced by Intel to relieve some problems with cable clutter, full sized expansion cards and processor cooling. A 20-pin plug (vs. the 12-pin AT plug) is also utilized which allows for additional power features.
Though ATX was initially introduced as the 'form factor of the future', it has been slow to be accepted, primarily because it requires a new case. ATX cases are generally more expensive than AT cases, and experience has shown that cooling is not appreciably better than the standard AT case with an intake fan. Today, form factor is primarily a matter of preference though ATX is becoming much more prevalent.
There are some variations on these two standards, such as 2/3 Baby AT or Micro ATX but the same case is used for all variations. The main difference between these and their full-sized cousins, other than the length, is the number of memory and expansion slots included. Many integrated motherboards use the Micro ATX form factor because the smaller PCB and fewer expansion slots allow for significant cost savings.
Full sized Baby AT boards are approximately 8.5 inches wide, and 10 or 11 inches long. The 2/3 and 3/4 Baby AT boards are the same width as the full Baby AT, but are an inch or two shorter. ATX motherboards are typically 12 inches wide, and about 8 inches deep, with the Micro ATX boards being as short as 5 inches.
Two additional form factors exist, called NLX and LPX, but these are used only by OEMs such as Compaq, Dell and others. Cases and motherboards using these form factors are not available to the general public except in complete systems. These motherboards can be identified by their single expansion slot, which takes a special 'riser' card. All PCI and ISA slots are on the riser card, with sound and video generally integrated on the motherboard. The limited capability and relatively low end components on these motherboards has helped to give a bad name to motherboard integration.