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Eight techniques, four families

There are several dozen named Sudoku techniques in the wild, but eight of them cover essentially every Easy, Medium, Hard and Expert puzzle ever printed, in four pairs: Hidden Single pairs with the plain naked single as the foundation; Naked Pair pairs with its mirror image Hidden Pair; Pointing Pair pairs with its mirror Box/Line Reduction; and X-Wing pairs with its three-line big sibling Swordfish and the chain-based XY-Wing. Everything else you'll see named on forums — Jellyfish, Skyscraper, Empty Rectangle, Coloring — is a renaming, a thickening, or a generalisation of one of these eight.

Naked Singles and Hidden Singles — the foundation

A naked single is a cell that has only one candidate digit left. It's the simplest move in Sudoku and the move that finishes most cells on the board. A hidden single is one step harder: it's a digit that has only one legal home inside a row, column or 3×3 box, even though the cell that will eventually receive it still carries many other candidates. The two together are the foundation of every solve. If you can scan all nine units for each digit 1 through 9 looking for hidden singles, you can finish any Easy puzzle without pencil-marks at all and most Medium puzzles with light marking.

Naked Pairs, triples and the candidate grid

The moment puzzles step past Medium, eyeballing stops being enough. You start needing pencil-marks — the small candidate digits in the corners of each cell. The first technique that pays for the cost of pencil-marking is the naked pair: two cells in the same unit whose candidate lists are both exactly the same two digits. Those two digits are locked into those two cells, so you can erase them from every other cell of the unit. Naked Pairs almost never place a digit directly; instead they trim candidate noise until a Hidden Single becomes visible on the next scan. Naked Triples and Naked Quads are the same idea with three or four cells and three or four shared digits. Its mirror image is the Hidden Pair: instead of two cells sharing the same two candidates, it's two digits confined to the same two cells — which may still carry other candidates that then get erased.

Pointing Pairs and Box/Line Reduction

A pointing pair uses the interaction between a 3×3 box and the row or column that runs through it. If, inside one box, a digit's only candidates all sit on one row (or one column), then that digit is forced to come from that box on that line — so it cannot appear elsewhere on that same line outside the box. Pointing Pairs (and their three-cell cousin, the Pointing Triple) appear in almost every Hard puzzle. The mirror technique, Box/Line Reduction, runs in the opposite direction: a line restricts a digit to one box, eliminating that digit from the rest of the box.

X-Wing and the fish family

The X-Wing is the gateway to advanced Sudoku and the first technique that asks you to think about the whole grid at once. The pattern is a rectangle of four cells in which a candidate digit appears in exactly two columns of two different rows. Whichever way the digit settles, it must take the two diagonal corners of the rectangle — so the digit cannot appear anywhere else in those two columns. Generalised to three rows, the same logic gives you a Swordfish; to four rows, a Jellyfish. Together these are called the fish family, and they all rest on the same counting argument the X-Wing uses. A different kind of advanced move, the XY-Wing, swaps counting for a three-cell chain: a pivot cell forces one of two "pincer" cells to hold a shared digit, no matter which of its own two candidates the pivot turns out to be.

How to learn in the right order

We strongly recommend learning Sudoku techniques in difficulty order rather than chasing the flashiest patterns first. The fastest route from beginner to confident solver is:

1. Internalise the rules (one minute) — see Sudoku rules.
2. Master Hidden Single on Easy boards until you can do a full scan in under a minute.
3. Add disciplined candidate pencil-marking and learn Naked Pair, then its mirror Hidden Pair, on Medium boards.
4. Learn Pointing Pair and Box/Line Reduction on Hard boards — most stalls break here.
5. Step into Expert territory with X-Wing, then its bigger sibling Swordfish and the chain-based XY-Wing.

Each step takes about a week of daily play to feel natural — roughly five Medium puzzles a day, with full pencil-marking. The compounding is real: by the time X-Wing clicks, Hidden Singles will feel instant.

Sudoku technique cheat-sheet

If you remember nothing else from this page, remember the rough order in which techniques appear on real boards. The first column is the technique. The second is the trigger you scan for. The third is the level of puzzle on which it usually first matters.

A short Sudoku vocabulary

Most search confusion about Sudoku comes from the names. Half the techniques are described with two or three different terms depending on which 1990s solver website coined them first. Here are the words that actually matter, in plain English.

• Cell — one of the 81 squares of the grid.
• Unit — a row, a column, or a 3×3 box. Each cell belongs to exactly three units.
• Candidate — a digit that could legally still go into a cell. Also called a pencil-mark or note.
• Given — a digit printed on the board at the start of the puzzle.
• Naked — describes a candidate list, e.g. a naked pair is two cells with exactly two candidates each.
• Hidden — describes a digit, e.g. a hidden single is a digit with only one home, regardless of how many other candidates the cell has.
• Locked candidates — the umbrella name for Pointing Pair and Box/Line Reduction.
• Fish — any technique in the X-Wing family (X-Wing, Swordfish, Jellyfish).
• Chain — a sequence of either-or links between candidates, used in advanced techniques like XY-Wing and Coloring.

How long should each level take?

Solving time varies by board and player. These ranges are rough orientation only; Zeloku's current labels use clue targets and an estimated complexity score rather than certifying a particular technique path.

• Easy — about three minutes once Hidden Singles feel automatic.
• Medium — five to eight minutes with disciplined pencil-marks.
• Hard — often ten to fifteen minutes, with more candidate tracking.
• Expert — often twenty minutes and up; the exact techniques vary.

FAQ

What's the difference between a Naked Single and a Hidden Single?

A Naked Single is a cell with only one remaining candidate digit. A Hidden Single is a digit with only one remaining candidate cell in its row, column or box, even though that cell might still have many candidates itself. Naked Singles are easier to see; Hidden Singles do most of the actual work in Easy and Medium puzzles.

Do I really need to pencil-mark?

Not for Easy or most Medium puzzles. From Hard upwards, pencil-marks aren't optional — without them, Naked Pairs and Pointing Pairs are essentially invisible. Every serious Sudoku apps often provide candidate tools for exactly this reason; on Zeloku, use Notes mode to enter them as you scan.

Is Sudoku really good for your brain?

The mainstream evidence says yes for working memory and pattern recognition, and probably yes for sustained attention. It will not, however, make you smarter at unrelated tasks — Sudoku is a habit, not a miracle. Five minutes daily beats one hour weekly.