Sudoku's are logic-based placement puzzles. The original games consist of 9x9 grids but can also be 4x4 grids or 6x6 grids. The goal of the puzzle is to enter a numerical digit from 1 through 9 in each cell 3×3 subgrids, starting with various digits given in some cells (the "givens"). Each row, column, and region must contain only one instance of each numeral. Completing the puzzle requires patience and logical ability. Variations on the puzzle are grids replacing the numbers with symbols or letters.