A sample of printing before the print job is started. It is the most accurate way to preview the output of a commercial print job. Also called a "contract proof," the colors approved by the customer in the proof are expected to be rendered identically in the finished job.Prepress proofs date back to the 1940s, where each of the CMYK colors were printed separately on film sheets and overlaid together to simulate the four-color printing process. Starting in the 1960s, 3M began developing proofing systems using various overlay methods, including its Color Key, Transfer Key and Matchprint brands. As digital printing technologies evolved, high-end inkjet printers began to produce output that was equally effective for proofs (see IRIS printer and Matchprint). See soft proofing and CMYK.