Key plate
Encyclopedia
In printing
Printing
Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....

, a key plate is the plate which prints the detail in an image.

When printing color image
Color image
A color image is a digital image that includes color information for each pixel.For visually acceptable results, it is necessary to provide three samples for each pixel, which are interpreted as coordinates in some color space...

s by combining multiple colors of ink
Ink
Ink is a liquid or paste that contains pigments and/or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing and/or writing with a pen, brush, or quill...

s, the colored inks usually do not contain much image detail. The key plate, which is usually impressed using black ink, provides the lines and/or contrast of the image.

However, in two-color images where neither color was black, the key plate might have been printed in the darker of the two colors.

Key plate is synonymous with keystone in the same context.
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