Normal sudoku rules apply, and cages show the sum of their digits and cannot include repeats. Each digit from 1-9 has a “buddy” which appears in the cell with its co-ordinates swapped – so if 1 and 4 are ‘buddies’, a 1 in r1c3 must be matched by a 4 in r3c1, and a 4 in r6c9 must be matched by a 1 in r9c6. This constraint applies to every digit in the grid. A digit’s ‘buddy’ can be itself.