Abstract: A structure wherein a hand held computer module can be conveniently and interchangeably coupled to accessory devices. The hand held computer module is detachably connected to a first accessory module by a first interlocking structure. The computer module includes the first interlock mechanism and the first accessory module includes the second interlock mechanism. The first and second interlock mechanisms are mated together to form a physical connection between the computer module and the first accessory module. The first accessory module also includes a third interlock mechanism which is similar to the first interlock mechanism. A second accessory module has a fourth interlock mechanism which is similar to the second interlock mechanism. The first accessory module can be detachably connected to a second accessory module by means of a second interlocking structure which includes the third and fourth interlock mechanisms.