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

The rules: insert the digits 1-6 just once in each a) row, b) column, c) bold outlined area AND d) in each white and gray rectangle.