In CMYK printing, registration black refers to 100% coverage in each of the four process colors: cyan (C), magenta (M), yellow (Y), and black (K). It is a special purpose color, and as such is not generally used to print black text or grayscale graphics (but see rich black).
Instead, registration black is used for printing crop marks, or "registration marks". When proofs for each color were generated on separate pieces of film, use of registration black made crop marks visible on all channels, providing a useful reference for alignment. A thin line printed in registration black can also be used to check whether the printing plates are lined up.
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