It would seem as that Microsoft liked the Dreamcast controller so much that they used it as the basis for the design of their own controller... Right down to the general shape, button layout, and even the two expansion ports (although they eliminated the window and replaced it with a very large Xbox logo.)
There were a some changes. Microsoft borrowed the idea for a second thumbstick from Sony's Dual Shock Controller, and added three extra buttons - the unnecessary seeming "back" button, and the inconveniently placed black and white buttons. The main face buttons were also made analog like on the Playstation 2 controller.
Unseen in the photo, Microsoft also added a "quick release" connector near the main connector at the end of the cable. The idea is that if someone trips on the cable, the quick release will snap apart rather than pull the heavy console off a table.
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