Here's the why? Most protractors assume that the cantilever is parallel to the side of the cartridge body and align the cartridge accordingly. That’s not always the case and even if it is, it can be difficult to get a clear view and know you’re sighting down the side of the cartridge correctly. Enter my mirror protractor. It relies on the principal of parallax to ensure that the sight picture is correct. Also the cantilever is aligned instead of the cartridge body, correcting any misalignment due to the cantilever being out of parallel with the cartridge body. All that is required is that the vertical alignment is correct. When sighting along the cantilever any misalignment will be seen as 3 lines - the scribed line, it's reflection and the reflection of the cantilever. When the cantilever, the scribed line and the eye are all in proper alignment there's just one line. No parallax = perfect alignment. They are used just like a paper 2 point protractor.
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There are other mirror protractors available some are painted or silk screened and the lines are so wide there's no reflection of a narrow cantilever. Others are just a decal stuck on the mirror. Mine are unique in that I hand scribe very fine alignment lines with a diamond into the glass mirror which is let into a plywood base. The nylon bushings are precision reamed to fit a Thorens spindle. They are made maintain accuracy and last indefinitely.
