Okay, in the old-style pointers, that is those using a 16-bit segment and a 16-bit address, you multiply the segment by 16 then add the address part to get the absolute address. This allows for the OS to locate the memory for a program anywhere in memory with a 16 byte granularity. In 286 mode, there are no checks for whether the proccess has permissions to use a given segment, so if you're accessing an absolute location in memory, it makes no difference if you access it with segment a+1 and address b or segment a and address b+16. When operating in 386 protected mode, however, the rules change.
Threaded Mode