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Sudoku techniques, from first steps to X-Wings

Twelve solving techniques, in the order you should learn them. Each guide has the rule in plain words, a real position from a puzzle, and the mistakes to avoid.

Every Sudoku can be solved by logic, and the logic comes in a small number of patterns. The easy ones place a digit directly. The harder ones remove candidates, which then lets an easy one place a digit. You do not need to know them all: the first three will finish any easy, medium or hard puzzle.

These are the same techniques the Sudoku Clan game uses to rate its puzzles and to explain its hints. In Duel, the turn-based mode, they are also worth points: the harder the technique behind a digit, the more it scores.

Beginner: placing digits

These three place a digit with certainty and need no pencil marks. Easy, medium and hard puzzles use nothing else.

Last empty cellA row, column or box has one empty cell left.Hidden singleA digit fits in only one cell of a box.Naked singleA cell has only one possible digit.

Intermediate: locked candidates

The first techniques that remove candidates. Expert puzzles need them.

Pointing pairsA digit's options in a box all sit on one line, so it leaves the rest of that line.Box/line reductionA digit's options on a line all sit in one box, so it leaves the rest of that box.

Advanced: pairs and triples

Groups of cells that share a small set of digits. Expert puzzles use pairs, Fiendish puzzles use triples.

Naked pairTwo cells in a unit share the same two options, so those digits leave the other cells.Hidden pairTwo digits fit in only the same two cells of a unit, so those cells lose every other option.Naked tripleThree cells in a unit hold only three digits between them.Hidden tripleThree digits fit in only the same three cells of a unit.

Master: fish and wings

Patterns that span the whole board. Only Fiendish puzzles need them.

X-WingA digit forms a rectangle across two rows and two columns.XY-WingA pivot and two pincers force a digit out of cells that see both pincers.SwordfishAn X-Wing across three rows and three columns.

Which technique do I need?

Work from the top. Before looking for anything advanced, make sure no single is left on the board, because nine times out of ten being stuck means a hidden single in a row or column has been missed. Then fill in pencil marks and check for locked candidates, then pairs, then triples, and only then fish and wings.

If you would like to be shown, the step-by-step solver names the technique for each move of any puzzle you enter, and the Hint button in the game does the same for the puzzle you are playing.

Techniques by difficulty level

Easy SudokuScan the boxes one digit at a time. No notes needed.Medium SudokuFewer clues. Scan rows and columns as well as boxes.Hard SudokuSome cells only fit one digit: check row, column and box together.Expert SudokuPointing, box/line reductions and pairs. Notes are a must.

Questions

What is the most important Sudoku technique?

The hidden single: finding the only cell in a row, column or box where a digit can go. It solves most of every puzzle at every level.

How many Sudoku techniques are there?

Dozens have been named, but about a dozen cover almost every puzzle you will meet in newspapers, books and apps. The rest are for specially built extreme puzzles.

Do I need advanced techniques to solve hard Sudoku?

No. In Sudoku Clan, puzzles up to Hard are solved with singles alone. Expert puzzles need locked candidates and pairs, and Fiendish puzzles need triples, X-Wings or Swordfish.

What technique should I learn after singles?

Pointing pairs. It is the first technique that removes candidates without placing a digit, and it unlocks most expert puzzles.

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