Final answer:
The slimline form factor similar to but not compatible with ATX is BTX.
Step-by-step explanation:
The slimline form factor similar to but not compatible with ATX is BTX.
BTX stands for Balanced Technology Extended and was developed by Intel as an alternative to ATX. While BTX has a similar layout to ATX, with the CPU and expansion slots in the same position, it has different dimensions and mounting points, making it incompatible with ATX cases and motherboards. BTX motherboards are shorter and wider than ATX motherboards.
On the other hand, ITX, Mini-ATX, and Micro-ATX are all compatible with ATX. ITX is the smallest form factor, Mini-ATX is slightly larger, and Micro-ATX is larger than Mini-ATX but smaller than full-size ATX.