1. A machine readable data file structure comprising: (a) at least two rows of codewords, one row being adjacent to and beneath another row; (b) each said codeword representing at least one information-bearing character and being selected from among a set of detectable mark patterns; (c) each of said mark patterns comprising a pattern of marks, each of said marks being representative of one of a plurality of different states; (d) the codewords in each said row being selected from a subset of said mark patterns, the subset for said row being defined by a specified rule for each of said rows that differs from the rule specified for said adjacent row; and (e) each said rule being a function of respective widths of said marks.