ATX Advanced Technology eXtended
The dominant PC form-factor spec since 1995 — motherboard 305 × 244 mm, standardised IO shield, power rails, PCIe slot placement.
Intel published the spec in 1995, replacing the AT form factor. Sub-formats: microATX (mATX) 244 × 244 mm, Mini-ITX 170 × 170 mm (for SFF builds), E-ATX 305 × 330 mm (HEDT/workstation), ATX12VO (Intel's proposal for a single-rail power connector, slow adoption). Backplate IO is standardised. Modern PC building is still bound by ATX — it shapes the choice of case, PSU, and cooler compatibility.