Numbers 1 to 9 are used
Sudoku is played on a grid of 9 x 9 spaces. Within the rows and columns there are 9 "squares" (made up of 3 x 3 spaces). Each row, column and square (9 spaces each) must be completed with the numbers 1 to 9, without repeating any number within the row, column or square. Sounds complicated?. The most difficult Sudoku puzzles have very few occupied spaces.