21x21

21x21 denotes a square grid consisting of 21 rows and 21 columns. The grid contains 441 individual square cells, arranged in a square with side length 21 units. It has 22 vertical and 22 horizontal grid lines, creating 484 lattice points at line intersections. The center of the grid is the cell in row 11, column 11 if indexing starts at 1. If each cell is a unit square, the overall width and height are 21 units, and the area is 441 square units.

In puzzles and games, 21x21 grids are used for nonograms (Picross) and as a standard size for some Sunday crossword puzzles, offering more space than smaller grids while remaining manageable. In Go variants, larger boards such as 21x21 are sometimes used for teaching or casual play, though the traditional Go board is 19x19. In computer science and mathematics, the 21x21 grid serves as a moderate-size test case for grid-based algorithms, tilings, and pathfinding problems.

Other uses include digital imaging and art, where a 21x21 pixel image comprises 441 pixels; color depth affects file size. The grid also appears in tiling and mosaic problems, where researchers study how to cover the grid with shapes under various constraints.