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. See Ritmeester's author page for his 18 other books. Peter Ritmeester is the inventor of the Hyper Sudoku, where four gray regions also contain 1-9. By applying this idea to a 6x6 sudoku, an entirely new dimension of logic is created, making a Sixy perhaps even more fun than a 9x9 sudoku. Not plain hard work, but plenty of logic and insight. Errors are unlikely, plus there are twice as many puzzles (and a six page How-To) in the same amount of space! In a Wacky Sixy few of the bold outlined areas are 2x3. This makes things much more difficult. Ritmeester, owner of PZZL.com, created the puzzles for all sudoku books presented by Will Shortz, with average reviews of 4.5 stars.