As you can see, there is a certain amount of redundancy in these typedefs. Some of this overlap is simply due to the history of the Windows APIs. The types listed here have fixed size, and the sizes are the same in both 32-bit and 64-applications. For example, the DWORD type is always 32 bits wide. Boolean Type BOOL is a type alias for int, distinct from C++'s bool, and from other types that repre