Key plate

In printing, a key plate is the plate that prints the detail in an image.

When printing color images by combining multiple colored inks, the colored plates typically do not contain much image detail. The key plate, most often printed in black ink, provides the main lines, shading, and contrast, giving definition and sharpness to the image. In instances where black is not used, such as in some early two- or three-color processes, the key plate may use the darkest available ink. In these cases, the term "key plate" may also be referred to as "keystone".