High Quality Content by WIKIPEDIA articles! A substitute check (also called an Image Replacement Document or "IRD") is a negotiable instrument used in the United States to represent the digital reproduction of an original paper check. As a negotiable instrument, a substitute check maintains the status of a "legal check" in lieu of the original check as authorized under the United States law Check Clearing for the 21st Century Act (Check 21 Act). Instead of presenting the original paper checks, financial institutions and payment processing centers electronically transmit data from substitute checks for settlement through the Federal Reserve System.