We should also mention to students that for 9 by 9, 16 by 16, and 25 by 25
Sudoku puzzles, each sub-grid is a square grid. For example, each sub-grid in a 9 by
9 Sudoku puzzle is 3 by 3, in a 16 by 16 Sudoku puzzle is 4 by 4, and so on. However,
there are also cases where sub-grids are not square. For instance, a 6 by 6 Sudoku
puzzle has six sub-grids, each of them being a rectangular 2 by 3 grid. Table 5
shows an example of 6 by 6 Sudoku master grids, where sub-grids are indicated by
bold grid lines.