Several compatible form factors are used for motherboards. The form factor refers to the physical dimensions and size of the board and dictates into which type of case the board will fit. The types of compatible industry-standard motherboard form factors generally available for system builders are as follows:

Obsolete form factors

• Baby-AT

• Full-size AT

• LPX (semiproprietary)

Modern form factors

• ATX

• Micro-ATX

• Flex-ATX

• NLX

• Mini-ITX (semiproprietary)

All others

• Proprietary designs (some Compaq, Dell Optiplex, Hewlett-Packard, notebook/portable systems, and so on); many Dell Dimension from 1996 through 2000 use ATX form factors, but with different electrical pinouts; newer Dell Dimension XPS systems use proprietary motherboards and power supplies as well.

The ATX motherboard design is far and away the most popular and best motherboard design for most people building their own systems today. It is also the most common. ATX is an open architecture that improves on the older Baby-AT design in many ways that affect other components of the computer, such as the case and power supply.

An ATX motherboard has the same basic dimensions as a Baby-AT; however, it is rotated 90° in the case from the standard Baby-AT orientation. This places the expansion slots parallel to the short side of the board, allowing more space for other components without interfering with expansion boards.

Components that produce large amounts of heat, such as the CPU and memory, are located next to the power supply for better cooling and easier access to the processor and memory sockets. ATX motherboards feature a high degree of port integration, but unlike Baby-AT motherboards, all ATX external ports are built into the motherboard and are mounted to one side of the expansion slots.

You won't need to use the clumsy and easily damaged or disconnected ribbon cables used by Baby-AT motherboards to carry mouse, serial, parallel, or USB ports to the rear of the system. The ATX-style power supply features a redesigned, single-keyed (foolproof!) connector that can't be plugged in backward or off-center.

Plugging power supply connectors backward or off-center by a pin is easy to do in the Baby-AT design, and the result is catastrophic: The motherboard is destroyed the moment the power is turned on. This is impossible with the shrouded and keyed ATX power connector.

This power supply provides the motherboard with the 3.3V current used by many of the newer CPUs and other components. ATX power supplies are also designed to support the advanced power-management features now found in system BIOSs and operating systems.

The micro-ATX form factor is a further development of the ATX design, intended for use in lower-cost systems. The micro-ATX architecture is backward-compatible with ATX and specifies a motherboard that is physically smaller, as the name implies. These smaller motherboards can be installed in standard ATX cases or in smaller cases specifically designed for the micro-ATX boards.

Slimline systems have used several form factors over the years. The LPX form factor was popular until the late 1990s, but the lack of standardization and the low-profile case used by most versions made it a very poor choice for system builders. Because of the differences among systems with LPX motherboards, upgrading the motherboard on a system that uses an LPX motherboard is very difficult.

The NLX form factor is another open standard Intel developed, with features comparable to the ATX but defining a Slimline-style case and motherboard arrangement. Systems based on the NLX standard should not experience the compatibility problems of LPX-based systems, but the inherent problems of the Low-Profile design remain.

These types of form factors are popular with corporate clients and novice consumers because of the systems' small footprint and the "everything-on-the-motherboard" design. However, replacement NLX motherboards are relatively scarce compared to the abundance of ATX and micro-ATX motherboards on the market.

Mini-ITX motherboards are a semi-proprietary design sold as a unit with one of several low-power-consumption VIA processors. Because the fastest C3 E-series processor used with Mini-ITX motherboards is just 1GHz, the mini-ITX form factor is unsuitable for high-performance systems.

However, because it features a PCI expansion slot and integrates almost all common I/O ports (even USB 2.0 and IEEE-1394a in the latest models), it's a good choice for a slimline system in which cooling and noise can be a problem. Many vendors now support Mini-ITX motherboards with low-profile cases and power supplies.

However, Mini-ITX is not a true industry standard because a complete specification is not available. Instead, it is just a marketing tool for VIA Technologies to sell some of its processors, chipsets, and motherboards; therefore, a limited number of motherboards, cases, and power supplies work with this format.

Mini-ITX has a potential for many interchange problems, especially with items such as power supplies. Unless having the smallest possible system is worth sacrificing upgradeability, repairability, and performance, you should choose a system based on the ATX standard instead.

You also need to verify that the motherboard supports both the processor you plan to install initially as well as the processor you might upgrade to in the future. For example, if you choose a Socket A motherboard, you should verify that it supports Athlon XP processors with 333MHz FSB, including the Athlon 2800+ and 3000+.

If you choose a Socket 478 motherboard, you should verify that it supports HT Technology (hyper-threading), a feature of the 3.06GHz and faster Pentium 4 processors. In some cases, a BIOS upgrade might be necessary. In addition to processor support and form factor, you should consider several other features when selecting a motherboard.