Size

• The original ATX motherboard, released in 1995, measures 12 inches wide and 9.6 inches deep, or 305 by 244 millimeters. The regular-sized ATX motherboard and the microATX, which was released two years later with a 9.6-by-9.6-inch -- 244-by-244-mm -- size, are the two most popular ATX variants. Others include the Extended ATX, or EATX, with a measurement of 12 by 13 inches -- 305 by 330 mm -- the Mini ATX, at 11.2 by 8.2 inches -- 284 by 208 mm -- and the FlexATX, at 9 by 7.5 inches -- 229 by 191 mm.
  
  

Processor and System Memory

• Each ATX motherboard dedicates a section for the placement of the PC processor via a central processing unit socket. This component is responsible for physically and electrically connecting the processor to the motherboard. The regular ATX has the CPU located at its upper middle section, while the microATX has it situated in its middle left area. Dual inline memory module slots -- usually numbering two, three or four -- are available for the PCs' system memory cards, which are based on Double Data Rate Synchronous Dynamic Random Access Memory architecture, or DDR SDRAM.
  
  

Disk Drive Connectors

• Each ATX motherboard has connectors, such as Serial Advanced Technology Attachment -- SATA -- or Integrated Drive Electronics -- IDE -- interfaces for the PC's disk drives. The latter usually consist of a hard drive for data storage, an optical disc drive for playing and recording CDs and DVDs and a floppy drive for using a floppy disk. The number of such connectors vary from one ATX variant to another. The microATX has four, for example, while the original ATX has three.
  
  

Expansion Ports

• Also included on each ATX motherboards are expansion ports, such as Peripheral Component Interconnect -- PCI -- and Accelerated Graphics Port -- AGP. These are used for installation of expansion cards, such as network cards, sound cards and graphics processing units. The number of ports, however, varies from one motherboard variant to the other. For instance, the regular ATX can have up to seven, the microATX, up to four, and the FlexATX, up to two.