Normal sudoku rules apply. Digits in column 1 indicate the column in which the digit 1 appears in that row (e.g. if r4c1 is a 6, r4c6 is a 1). The same applies to columns 5 and 9, with the digits 5 and 9 respectively. Each digit in the grid represents a skyscraper with a height corresponding to its value. Taller skyscrapers block smaller skyscrapers from view. Clues outside the grid indicate how many skyscrapers can be seen, in total, when looking along that row or column from that direction.