Normal sudoku rules apply (ie place the digits 1-8 once each into every row, column and box). The digit in a lettered cell indicates exactly how many times that digit appears in a cell containing that letter. Eg: a 5 in a C cell indicates that there are exactly five 5s in cells containing the letter C. However, 8 of the lettered cells are naughty; these don’t obey the letter rule. Eg: a 5 in a naughty C cell indicates that there are NOT exactly five 5s in cells containing the letter C. Every row, column and box contains exactly one naughty lettered cell.