Sudokurules.

A sudoku grid is a 9×9 square, split into nine 3×3 boxes by thicker borders. Some cells start filled in — those are givensand you can’t change them. Your job is to fill in every empty cell with a digit 1-9 such that:

1. Every row contains each digit 1-9 exactly once.
2. Every column contains each digit 1-9 exactly once.
3. Every 3×3 box contains each digit 1-9 exactly once.

That’s all of them. A valid solve fills every empty cell so all three rules hold simultaneously. There’s no scoring built into the rules; that comes from individual implementations (here’s how Melio’s works).

• Not math. The digits 1-9 are symbols. You never add, multiply, or compare them. Sudoku works identically with letters or colors — people just standardized on digits.
• Not a guessing game. Every cell placement should be logically deducedfrom the existing clues. If you’re guessing and hoping, the puzzle has more clues to use; you just haven’t found them yet. (Or, on Extreme puzzles, you need an advanced technique like an X-wing — see the techniques guide.)
• Not solvable by trial and error in a reasonable time. 9! = 362,880 ways to fill one row. The grid has 9 rows, so brute-force is 10^21+ combinations. Sudoku is solvable by humans because the constraints prune the space dramatically — but only if you reason about them, not try them.

A puzzle is solved when:

• Every cell holds a digit 1-9 (no blanks).
• No row, column, or 3×3 box contains a duplicate.

A well-formed sudoku has exactly one valid solve. If a puzzle has multiple valid solutions, the constructor messed up — a sudoku is supposed to be uniquely solvable by deduction. Every Melio puzzle is uniqueness-checked before it’s served, so you can trust that the solve you find is THE solve.

Sudoku as we know it was popularized in Japan in the 1980s under the name “Sūji wa dokushin ni kagiru” (“the numbers must be single”), shortened to sudoku. It went global in 2004 when British newspapers started printing daily grids.

The underlying mathematical structure is older: it’s a "Latin square"(an n×n grid where each row and column has each symbol exactly once) with the added 3×3 box constraint. Leonhard Euler studied Latin squares in the 18th century, but didn’t add the box rule.

In 2012, mathematicians at University College Dublin proved that the minimum number of clues for a uniquely solvable sudoku is 17. Melio’s Extreme difficulty uses exactly 17.

The classic 9×9 sudoku has many variants. Melio plays the standard rules; we don’t ship variants yet. Common ones you might hear about:

• Killer Sudoku: cells are grouped into irregular “cages” with target sums; in addition to the standard rules, the cells in a cage must sum to the target without repeats.
• Hyper Sudoku: adds four extra 3×3 inner regions that also need to contain 1-9 — same grid, more constraints.
• X-Sudoku (Diagonal Sudoku): both main diagonals must also contain 1-9 exactly once.
• Mini-Sudoku (4×4 or 6×6): smaller grids, same kind of rules. Good for kids or quick breaks.
• Samurai Sudoku: five overlapping 9×9 grids forming a + shape. The center 3×3 boxes are shared across grids.