Sudoku
Classic number placement
9×9 grid · rows, columns, and 3×3 boxes
Goal
Fill the 9×9 grid so every row, every column, and every 3×3 box contains the digits 1–9 exactly once.
Rules
- Each row holds 1–9 with no repeats.
- Each column holds 1–9 with no repeats.
- Each of the nine 3×3 boxes holds 1–9 with no repeats.
- Some cells are given; deduce the rest — never guess.
How to play
- Scan rows, columns, and boxes for a digit that can go in only one place (a hidden single).
- Pencil in candidates for the tricky cells and narrow them with the solving techniques.
- Every GridDay puzzle has exactly one solution reachable by pure logic.
Tip
Start with the digit that already appears most often on the board — it is the easiest to place next.
Techniques to learn
Naked Single · Hidden Single · Last Candidate · Naked Pair · Hidden Pair · Pointing Pair · Box-Line Reduction · Naked Triple · Hidden Triple · X-Wing · Y-Wing · XYZ-Wing · Skyscraper · Two-String Kite · Empty Rectangle · Naked Quad · Hidden Quad · W-Wing · Swordfish · Jellyfish · Finned Fish · Simple Coloring · X-Chain · XY-Chain · Inference Chain · ALS-XZ