Extended ATX

From PC Jargon Dictionary
ATX motherboard size comparison; rear is on left.
  FlexATX (229 × 191 mm)
  microATX (244 × 244 mm)
  Mini ATX (284 × 208 mm)
  ATX (305 × 244 mm)
  Extended ATX (EATX) (305 × 330 mm)
  WTX (form factor) (356 × 425 mm)

Abbreviated as EATX or E-ATX, extended ATX is a wider form of ATX. The official specification for E-ATX is 305 x 330mm (12 x 13in), which is the same as SSI-EEB, but the width varies depending on the manufacturer and model of motherboard.