What form factor was designed as a replacement for the ATX but was never produced?

The Information Technology eXtended (ITX) form factor was developed with the intention of addressing the needs for compact and efficient system designs but never entered mass production as a direct replacement for the ATX form factor. While various other form factors like Micro-ATX and Mini-ITX were indeed produced successfully, ITX primarily served as a conceptual platform that influenced smaller motherboards catering to embedded systems and compact cases.

This highlights its significance in demonstrating the evolution of motherboard design, although it ultimately did not achieve large-scale adoption as a standardized form factor, which distinguishes it from others that are widely utilized today. The other mentioned choices—Micro-ATX, Mini-ITX, and Mini-ATX—are all established and produced form factors that meet different design needs, making them distinct from ITX, which was intended but not realized in a commercial capacity.