applications. Other smaller BTX sizes include: microBTX at and nano BTX at . The heat sink to be attached to the CPU, called "Thermal Module" throughout the official specification, is no longer attached solely to the motherboard, but to the casing itself, so that the inertial load of its mass during a mechanical shock event can no longer damage the motherboard. 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. Compatibility with ATX products In the first months of production the ATX and BTX motherboards were so similar that moving a BTX motherboard to an ATX case was possible and vice versa. This was possible because the first BTX motherboards were ATX motherboards turned upside down, except for the component location that really were BTX positioning. Later the BTX form factor had a big change by turning it into a mirror image of the ATX standard. Since the new motherboard design, both standards are incompatible. Basically BTX motherboards are 'leftside-right' compared to ATX and not upside-down as before: i.e. they are mounted on the opposite side of the case. Some computer cases such as the Cooler Master Series (Stackers) were released to support a varying range of motherboard standards such as ATX, BTX, Mini-ATX and so forth, to ease motherboard upgrade without buying a new case; however, all connector and slot standards are identical, including PCI(e) cards, processors, RAM, hard drives, etc. BTX power supply units can be exchanged with newer ATX12V units, but not with older ATX power supplies that don't have the extra 4-pin 12V connector, which was introduced with the ATX12V standard. Reception The BTX form factor has not been widely adopted despite its improvements over ATX and related standards. As a result, the availability and variety of BTX-compatible components is limited. One reason for the failure of BTX to gain traction in key markets was the rise of energy-efficient components (CPUs, chipsets and GPUs) which require less power and produce less waste heat, eliminating two of the primary intended benefits of BTX. Another reason was the lack of OEM adopters. Initially, only Gateway and Dell offered computers with the new