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Many watches (especially swiss) use a left-hand screw on the crown wheel because the crown wheel only rotates counter-clockwise, and in simpler designs it would be possible for the friction between the edge of the crown wheel and the screw to actually unwind the screw. Of course, most of the better designs use some form of center post that isolates the two, but the tradition continues.

All in all, it can be a bit of crap-shoot deciding if the crown wheel screw is left or right threaded, and I'm sure many a screwhead has been sheared off by confused watchmakers faced with a tight screw on an unfamiliar movement. Some manufacturers use the helpful "triple notch" on left handed screws, but not all. In some rare cases, I've actually seen left handed screws used for both the crown and ratchet wheel.
 
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