Pocket Posh Sixty Sudoku Hard 6x6 Puzzles with a Twist

Description

Peter Ritmeester, founder and CEO of PZZL.com, is the inventor of Hyper Sudoku, a Sudoku puzzle that has four gray regions that also contain the digits 1 to 9. By applying this idea to a 6 x 6 Sudoku, an additional level of logic is created. Pocket Posh Sixty Sudoku Hard features a foreword, an extensive how-to-play section, 200 hard puzzles, and complete solutions at the back.

The rules: insert the digits 1 to 6 just once in each row, column, bold outlined area, and in each white and gray rectangle.