dailysudoku.com

[codebreaker]

I have noticed that all the Sudokus I have seen so far have numbers in the same location when the puzzle is turned upside-down. Allthough the numbers are different, their location on the grid is identical. This holds true for the Pappocom puzzles I have been doing in the Toronto Star newspaper and also for a book I have containing 300 Sudokus. Can someone tell me why this is the case? I have found Sudokus easier to solve if I work on the corresponding boxes first, i.e. the lower right box corresponds to the upper left, and so on.

[TKiel]

It's called symmetry and while there are many different kinds, most Sudoku puzzles use (I think) 180 degree rotational. It's not necessary for a puzzle to be symmetrical for it to be valid, but when entering a starting grid manually it makes it easier to catch either omitted or misplaced numbers. It's an additional constraint that some puzzle setters impose on themselves, plus the initial clue set can have an interesting look to it.

[David Bryant]

There's also the fact that crossword puzzles are traditionally constructed with 2-fold rotational symmetry, so puzzle editors are used to seeing such patterns. dcb