ATX is a motherboard specification developed by Intel in 1995. It defines the dimensions, mounting points, and power connections for motherboards, ensuring compatibility with cases and power supplies. The standard size for an ATX motherboard is 305 mm x 244 mm (12 in x 9.6 in), allowing for multiple expansion slots and ports. The ATX design has led to various derivatives, including MicroATX and Mini-ITX, which are smaller and cater to different needs. These variations maintain the same mounting points and power connections, making it easier for users to upgrade or change components while ensuring compatibility across different systems.