How to play Sudoku

The rules

• The grid has 81 cells in nine rows, nine columns and nine 3×3 boxes.
• Fill every empty cell with a number from 1 to 9.
• Each row, column and box must contain every number exactly once.

That’s all. A proper Sudoku has exactly one solution, and you can reach it without guessing.

Where to start

Pick a number that already appears often and check each box that’s missing it. Rows and columns that already hold that number rule out cells in the box. When only one cell is left, place it. Repeat for each number, then look for rows or boxes with only one or two gaps.

Techniques, easiest first

Singles

Naked single. A cell where only one number fits, because the other eight already appear in its row, column or box.

Hidden single. A number that has only one possible cell left in a row, column or box. It goes there, even if that cell has other candidates.

These two solve every Easy puzzle.

Eliminations

Pointing. If every candidate for a number inside a box sits in one row, that number must go in that row within the box. Remove it from the rest of the row. The same works for columns.

Box/line reduction. The reverse: if every candidate for a number in a row falls inside one box, remove that number from the other cells of the box.

Naked pair. Two cells in the same row, column or box with the same two candidates, say 4 and 7. Those two numbers must go in those two cells, so remove 4 and 7 from every other cell in that unit. Triples work the same way with three cells.

Hidden pair. Two numbers that can only go in the same two cells of a unit. Those cells can hold nothing else, so clear their other candidates.

You’ll need these on Medium and Hard.

Patterns

X-Wing. If a number can go in only two cells in each of two rows, and those cells line up in the same two columns, the number must sit on opposite corners of that rectangle. Remove it from every other cell in those two columns. Swap rows and columns for the mirror case.

XY-Wing. A pivot cell with candidates X and Y sees one cell with X and Z and another with Y and Z. Whichever value the pivot takes, one of those two cells becomes Z. Any cell that sees both of them can’t be Z.

These show up on Expert.

Using notes

Notes are small candidate numbers you pencil into a cell. Tap Notes or press N, then tap numbers to add or remove them. You can also long-press a number key to add a single note without switching modes. When you place a number, Best Sudoku removes it from the notes in the same row, column and box. Turn on Positional note keys in Settings if you want each key to sit where its note appears in the cell.

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