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The size of the system case is often determined by the motherboard form factor. The following table contains the primary motherboard types with which you should be familiar:

Form Factor Characteristics
ATX The ATX form factor is the most common form factor for full-sized computers.
  • ATX boards measure 12" x 9.6".
  • The CPU sits at the back top below the power supply. The power supply blows air into the case or pulls air from the case to cool the processor.
  • The power cable runs from the system case power switch to the system board.
  • The power supply can use a soft switch or soft power (the operating system can turn the computer off).
Mini-ATX A mini-ATX motherboard is a slightly smaller variation of the full ATX size that measures 11.2" x 8.2".
  • The main difference between ATX and mini-ATX is the number of bus and possibly memory slots on the motherboard.
  • Mounting holes for both are located in the same place, making them interchangeable in most cases. A case that supports an ATX motherboard can also support a mini-ATX motherboard.
Micro-ATX The micro-ATX form factor is an even smaller version of the ATX standard, with a maximum size of 9.6" x 9.6". Mounting holes are in the same position as ATX motherboards. Note: The terms mini-ATX and micro-ATX are often used interchangeably, and some micro-ATX boards could be smaller than the maximum size.
Mini-ITX The mini-ITX standard is the smallest variation of the ATX standard, with a maximum motherboard size of 6.7" x 6.7". The mini-ATX standard also includes standards for a power supply that provides less than 100 Watts.
NLX NLX is an older form factor used for slimline desktop-style computers. NLX is an improvement over an even earlier LPX form factor. NLX:
  • Uses a riser card in the middle of the system board (expansion slots are located on the riser card rather than the system board).
  • The riser card does not have built-in ports for audio, joystick, USB, network, and modem.
  • Supports AGP video cards.
  • Includes the ability to mount the motherboard so it can slide in or out of the system case easily.
BTX The BTX form factor was designed to give developers better options for managing system performance and balancing thermal management. With BTX:
  • The processor is at the front and turned on an angle to increase air flow across the processor.
  • A thermal module or shroud fits over the processor to move heat directly out of the system.
  • Many BTX cases are also ATX compatible.
Although BTX was developed as an improvement to ATX, it has not gained widespread adoption. BTX is implemented mainly by computer manufacturers such as Dell.

System cases come in the following basic types:

Type Description
Desktop Desktop cases sit horizontally and are usually used for low-end systems that are not meant to be upgraded (i.e. there may be few or no expansion slots). Specific sizes include:
  • Desktop
  • Slim line
Tower Tower cases can be as high as two feet tall. They have extensive room for expansion. Tower size classifications include:
  • Minitower typically have 1-2 drive bays for expansion
  • Midsize typically have between 2-4 drive bays
  • Full-size may have between 6-10 drive bays for expansion
Small form factor (SFF) Small form factor computers use mini-ITX or custom motherboards to reduce the size of the system. Some SFF cases might be classified as minitowers with 1-2 drive bays. Others might include all computer components in the shape of a ball or integrated within a monitor as a single unit.
Notebook Notebook cases are generally proprietary and often vary among models.

When you purchase a system case, you will typically get the following components:

  • System case
  • Power supply (although the power supply might also be separate)
  • Case fans
  • Plastic or rubber feet that attach to the bottom of the case
  • Metal screws for attaching the motherboard
  • Additional external connectors (such as audio, USB, and Firewire) that connect to motherboard headers