**Sudoku** (also spelled **Su Doku** and originally called **Number Place**) is a game of logic, often played with pencil and paper, where the object is to fill a grid of spaces with a set of symbols (usually numbers or letters) such that each symbol only appears once in each grid subdivision.

The original modern Sudoku problem was called "Number Place" and featured a nine-by-nine grid where the object was to fill in each of the numbers 1 through 9 such that each number appears exactly once in each row, column and three-by-three sub-grid. Later versions of the Sudoku puzzle used different-sized square grids and overlapping grids. Other variations include irregularly shaped subgrids.