In the image assembly phase of prepress, one of two separate flats which will be imaged on the same printing plate. Complementary flats are one means of handling disparate page elements which need to be imaged on the same plate, but which yield better results when imaged separately on the same plate (such as two images which need to butt each other, or need to overlap slightly). For example, in the case of abutting images, the space between the adjacent images may be too narrow to make an effective splice on the same flat. The process of using complementary flats in platemaking is called double printing. If images on each complemenatry flat are to overlap, it is known as surprinting.
In some cases, separate images can be prepared on complementary flats and combined photographically onto a single film prior to platemaking, a process called photocombining. Such a negative (or positive) is called a composite negative (or composite positive). (See Prepress: Graphic Arts Photography and Flat Assembly.)