A motherboard's actual measurements, connection positioning, I/O interfaces, and compatible power sources are all specified by the ATX standard. In order to supplant the prior AT standard for desktop Computers, Intel developed the ATX specification. Since then, a number of ATX versions have been released and are widely used in desktop PCs.

The motherboard architecture was significantly improved by the ATX standard in a number of ways. It immediately integrated connections into the processor and defined where the I/O panel was located. Additionally, it relocated the Processor and Memory ports so that they were out of the way of full-length extension devices. It specifies that suitable power sources must have a number of connections to supply electricity to the system, CPU, and extension devices. In order to reduce the length of the necessary cords, it relocated the connection for storage devices closer to where the drive ports were located. Additionally, ATX processors and enclosures allow for more efficient ventilation through the frame than earlier models.

Any ATX chipset will work in an ATX case because the ATX standard specifies where the attachment slots should be on a computer chassis. Additionally, it defines a range of sizes, some of which are bigger to accommodate more extension connections and others of which are smaller to suit in tiny cases:

• FlexATX – 9 × 7.5 in (229 × 191 mm)
• MicroATX – 9.6 × 9.6 in (244 × 244 mm)
• ATX — 12 × 9.6 in (305 × 244 mm)
• Extended ATX (EATX) – 12 × up to 13 in (305 × up to 330 mm)