BTX (form factor)

It was designed to alleviate some of the issues that arose from using newer technologies (which often demand more power and create more heat) on motherboards compliant with the circa 1996 ATX specification.

The first generation of Apple's Mac Pro used some elements of the BTX design system as well, but was not BTX-compliant, instead using a proprietary form factor.

These motherboards share a common top half with the other sizes in the BTX line, but support only one or two expansion slots, designed for half-height or riser card applications.

The structural interface between the heat sink and the chassis, is defined as 4 mounting holes with the distances of 4.4 × 2.275 in (55.79 × 111.76 mm) between one another.

And since this attachment means is also required to have a certain stiffness, it is called "Support and Retention Module (SRM)" in the specification.