Sudokuisplayedovera9x9grid,dividedto3x3subgrids.
Sudokubeginswithsomeofthegridcellsalreadyfilledwithnumbers.
TheobjectofSudokuistofilltheotheremptycellswithnumbersbetween1and9.
Numbercanappearonlyonceoneachrow,andnumbercanappearonlyonceoneachcolumn.
Also,numbercanappearonlyonceoneach3x3subgrids.
Thedifficultycanbeselectedbythreestages.