Since the new motherboard design, both standards are incompatible. Later the BTX form factor had a big change by turning it into a mirror image of the ATX standard.
This was possible because the first BTX motherboards were ATX motherboards turned upside down, except for the component location that really were BTX positioning. 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.
