Fixed-width files from mainframe systems are one of those topics where there's very little written about the specifics and a lot of institutional knowledge living in people's heads or in COBOL copybooks that haven't been updated since 2003. If you're new to a financial organization and someone hands you a layout spec for a mainframe extract, this is for you.What Makes Mainframe Fixed-Width Files DifferentUnlike CSV, there are no delimiters. Each field occupies a fixed byte position in the record. A customer ID always occupies bytes 1-10, an account balance bytes 11-22. To read the file correctly, you need the layout spec: a document that maps field names to byte positions and data types. If the layout spec is wrong or out of date, you will read valid-looking garbage.