Abstract: A machine readable bar code is formed from a bar code datum by generating a first bar code image section encoding the bar code datum by a horizontal arrangement of first vertical bar sections separated by first vertical space sections wherein the first vertical bar sections have first widths selected to produce a first printed bar code section of correct ratios of space width to bar width for a first type of printer; and generating a second bar code image section encoding the bar code datum by a horizontal arrangement of second vertical bar sections separated by second vertical space sections wherein the second vertical bar sections have second widths larger than the first widths and selected to produce a second printed bar code image section of correct ratio of space width to bar width for a second type of printer. A document containing a print of this dual bar code is then readable by a conventional bar code scanner irrespective of the type of printer used to print the document.