An old-time traditional animator's term, a "Key" is a thumbnail of a character or a background element with color reference.  Acme's Character Keys are composed of a cel layer floated over a lithographic background.  The cel features a color rendition of the character while the background features sketches and color reference.   Each character key comes matted with a title tag and a certificate of authenticity.  Matted measurements are 8 inches by 6 inches.