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The best-known sudoku type: on a 9x9 grid, the digits 1 to 9 each appear once in every row, column, and 3x3 box.

6x6 mini sudoku is a scaled-down classic sudoku: digits 1 to 6, played across six 3x2 boxes. Ideal for beginners and kids.

4x4 mini sudoku is the easiest way to learn sudoku logic. Digits 1 to 4, across four 2x2 boxes — you can finish it in a couple of minutes.

12x12 big sudoku is a larger, tougher version of classic sudoku. Digits 1 to 12, played across twelve 4x3 boxes.

16x16 giant sudoku is the largest and toughest form of classic sudoku. Digits 1 to 16, played across sixteen 4x4 boxes.

Word Sudoku follows the exact same rules as classic 9x9 sudoku; the only difference is that nine letters, A through I, replace the digits.

Diagonal sudoku (X-Sudoku) adds one extra layer to the classic 9x9 rules: both main diagonals of the grid must also contain the digits 1 to 9 exactly once.

Windoku (sometimes called Hyper Sudoku) adds four shaded "window" regions to the classic rules; each of these regions also contains 1-9 once.

In jigsaw sudoku, irregularly shaped nine-cell regions replace the standard 3x3 boxes. Row and column rules stay the same, but you'll need to trace the region borders visually.

Argyle sudoku features shaded diagonal bands forming a diamond (argyle) pattern across the grid. Each of these bands also contains the digits 1-9 once.

Asterisk sudoku has a star/asterisk-shaped set of nine cells marked near the center of the grid. Those cells must also contain the digits 1-9 once.

This variant brings chess logic to the classic rules: a digit can't repeat in any cell a knight's move away, and it also can't repeat in any diagonally or orthogonally adjacent cell (a king's move).

Killer sudoku blends classic sudoku logic with simple arithmetic. No digits are given at the start — only the sums of dashed-outline "cages" are shown.

In sandwich sudoku, the numbers along the edges of rows and columns show the sum of the digits "sandwiched" between the 1 and the 9 in that row or column.

In frame sudoku, numbers surround all four sides of the grid; each one shows the sum of the three cells of that row or column nearest that edge.

Little killer sudoku gives no digits at all. Instead, arrow-marked numbers sit along the grid's edge; each one is the sum of the diagonal of cells the arrow points along.

In quadruple sudoku, a circle sits at the corner point where four cells meet; the digits in that circle must appear somewhere among those four cells (position unspecified).

In skyscraper sudoku, each cell's digit represents the height of a skyscraper at that spot. The edge number shows how many skyscrapers are visible looking in from that direction (shorter buildings hidden behind taller ones don't count).

In odd/even sudoku, certain cells carry a special mark: circle-marked cells must hold an odd digit, square-marked cells must hold an even digit.

In consecutive sudoku, a small dot between two neighboring cells means those cells' digits are consecutive (differ by exactly 1).

In Kropki sudoku, dots placed between neighboring cells give extra clues: a white dot means the two cells are consecutive, a black dot means one is double the other.

Samurai sudoku consists of five 9x9 grids linked at the corners. The four corner boards each share one 3x3 box with the center board. That's 369 cells total — a real marathon puzzle.

Twin sudoku consists of two 9x9 grids linked at one corner. The two boards share exactly one 3x3 box. Shorter than samurai sudoku, but still a longer challenge than a single standard board.