Key plate

From Freepedia

In printing, a key plate was the plate which printed the detail in an image.

When printing color images by combining multiple colors of inks, the colored inks usually did not contain much image detail. The key plate, which was usually impressed using black ink, provided the lines and/or contrast of the image.

However, in two-color images where both colors were not black, the key plate might print in the darkest of the colors.