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Additive primary colors - The three main color regions--red, green, and blue--that each represent about a third of the visible light spectrum.  Combining all three of these colors of light creates white light.

Caldecott Medal - An annual award presented by the American Library Association to the illustrator of the most distinguished children's picture book of the previous year.

Camera separation - The process by which full-color art is separated into four monochrome images, one for each of the process colors of ink, in order to reproduce the colors of the original when overprinted together. This process involves photographing the original art four times through special filters. A halftone screen is also used to capture gradations in color.

Color separations - Specially prepared artwork that has isolated individual colors of ink for printing. Typically there would be one color separation for each color of ink. Color separations can be created through a photographic process but children's book illustrators were often asked to manually create the color separations-- i.e., to provide preseparations.

Continuous tone copy - Copy with a range of tones, such as a pencil drawing or photograph. Continuous tone copy must be converted to a single-tone image through the use of a halftone screen in order to be reproduced on a printing press.

Copy - Any material to be printed.

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Flat tint - Lightened shades of any printing ink made using uniform halftone dots. 

Four color process printing - A method of full color printing that uses only four colors of ink: cyan, magenta, yellow, and black. These inks can be overprinted to create nearly any color in the visible spectrum.

Halftone - A method for reproducing continuous tone copy that involves photographing the copy through a fine screen of intersecting lines. This creates an image consisting of thousands of tiny dots that create the illusion of varying tones. Halftoning effectively converts continuous tone copy into a single tone image that can be reproduced on a printing press.

Key plate - The piece of copy that serves as the master preseparation. The key plate usually represents the black color of the picture and contains most of the detail. Additional color separation are added to the key plate on overlays.

Line copy - A type of copy that consists of solid lines and areas of color with no gradations in tone.

Lithography - A type of planographic printing that uses contrasting ink-receptive and water-receptive areas on a stone or metal plate to print from a flat surface.

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Offset lithography - The commercial form of lithographic printing utilizing rotating cylinders for rapid reproduction. The image to be printed is offset from the printing plate onto a rubber blanket cylinder before being transferred to the paper.

Overlays - One or more pieces of copy meant to “lie over” the key plate and indicate an additional color to print. Overlays were generally executed on acetate or thin sheets of paper and often included additional written instructions for the printer.

Overprinting - The process of printing two or more transparent inks on the same surface to produce a new color. Overprinting is central to four color process printing.

Planographic printing - A method of printing in which the printing plate is a flat surface.  The most commonly used method of planographic printing is lithography, where the contrast between the image area and the non-image area is created through a chemical process involving oil and water.  

Preseparation - Art for which the areas to be printed with different colors of ink are manually separated by the artist before being sent to the camera person to prepare for printing.

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Printing press - A machine designed for the rapid reproduction of lettering or images by contact between inked plates and paper or other materials.

Process colors - The subtractive primary colors: cyan, magenta, and yellow, along with black. Process color inks can be overprinted to create nearly any color in the spectrum.

Register marks - Markings placed on the key plate and each overlay in precisely the same spot to show the platemaker how these pieces should be arranged in relation to one another to create a single image.

Relief printing - A method of printing in which the printing plate has raised surfaces that transfer ink. Common relief printing methods include letterpress and woodcuts.

Separation negative - A negative image created by photographing a piece of full color artwork through a colored filter. This negative shows the distribution of a single process color as found in the original copy and is used to make a printing plate for that color of ink. A separation negative is the result of camera separation and is not a form of preseparation.

Subtractive primary colors - The three colors--cyan, magenta, and yellow--that are created by removing one of the additive primary colors from white light.