Sudoku
There are many Sudoku variations that have been created since the original Sudoku's release.  The grid is most often 9 by 9, but can be different sizes, though, and separated into smaller grids in different ways. For example, there is a 6 by 6 grid with dark lines between the third and fourth columns and between two pairs of rows: the second and third rows, and the fourth and fifth rows. The numerals that must be filled in here are from 1 to 6.