Frame Sudoku (also known as "Outside Sum Sudoku" and "Sum Frame Sudoku") consists of an empty grid. Numbers in the outside frame equal the sum of the first three numbers in the corresponding row or column in the given direction.
Sudoku may contain additional four regions 3 x 3. This format was first introduced in the Dutch newspaper "NRC Handelsblad" by Peter Ritmeester. It is called Windoku ("Four-Box Sudoku", "Hyper Sudoku").
