Solving techniques
Sudoku techniques, basic to advanced
There are dozens of named Sudoku techniques and you need about six of them. These are the ten worth knowing, in the order they start paying off — from the basic singles that finish an easy grid to the advanced patterns an expert board is built to demand. Each one comes with an interactive board that walks through a real example rather than a diagram.
- 01Naked singlesEvery boardA cell with one candidate left, forced by its row, column and box together. The first thing anyone works out on their own — worth naming because knowing where to look for them is what makes a solve fast.
- 02Hidden singlesEvery boardA digit with exactly one home left in a row, column or box, in a cell that still looks unsolved. Learn this one properly; it finishes most easy and medium grids by itself.
- 03Naked pairsMedium and upTwo cells in a unit holding the same two candidates lock both digits away from everything else in that unit. The first technique that eliminates rather than places.
- 04Hidden pairsMedium and upTwo digits with only two homes between them in a unit. The mirror of a naked pair: the eliminations land inside the two cells rather than around them.
- 05Pointing pairsHardA digit confined to one line inside a box is eliminated from the rest of that line. Cheap to check and it unlocks more hard boards than anything else on this list.
- 06Box/line reductionHardThe same interaction read the other way: a digit confined to one box within a line leaves the rest of the box. Run it alongside pointing pairs for a complete pass.
- 07Naked triplesHardThree cells whose candidates union to three digits. No cell has to show all three, which is exactly why they get walked past.
- 08X-WingHardFour cells at the corners of a rectangle let you strip a digit out of two whole columns. The first pattern that reasons across the grid instead of inside one unit.
- 09XY-WingExpertThree two-candidate cells in a chain. Whichever way the pivot resolves, one wing must hold the shared digit — the first technique that argues by cases.
- 10SwordfishExpertThe X-Wing idea over three lines instead of two. The logic is the same; spotting it is far harder, because it rarely looks like a pattern at all.
Which of these count as advanced
“Advanced” is not a formal grade, but in practice it means the techniques past the pairs: X-Wing, XY-Wing and Swordfish. What separates them from everything above is scope. Singles, pairs and triples are arguments about one unit; the advanced patterns hold two or three lines against each other and eliminate across the whole grid.
The two box/line interactions — pointing pairs and box/line reduction — sit on the border and are the ones worth learning first if you are here because a hard board stalled. They cost seconds to check and they unlock far more grids than an X-Wing does.
Why this order
Techniques are usually listed by difficulty, which is the wrong axis. What matters is how often a technique earns its keep, and that falls off a cliff after the first two.
Hidden singles and scanning finish the large majority of easy and medium puzzles on their own. Pairs handle most of what is left, and the two box/line interactions — pointing pairs and box/line reduction — unlock the bulk of hard boards for very little effort. X-Wing, XY-Wing and Swordfish exist for grids specifically built to defeat everything simpler, which is to say you will use them on expert boards and almost nowhere else.
Learning them out of order is the classic mistake. Somebody reads about X-Wings, starts hunting rectangles on a medium grid, and misses the hidden single that was sitting in row three the whole time.
Candidates first
Everything past hidden singles is expressed in terms of candidates — the digits a cell could still hold. You cannot see a naked pair, an X-Wing or a Swordfish without them written down.
This is not cheating and it is not a crutch; it is how the techniques are defined. The board on this site has a notes mode for exactly this, and so does the app. The only place candidate marking is a genuine trade-off is a timed match, where the seconds it costs have to be weighed against the cells you lose while marking.
The one rule underneath all of them
Every technique on this list is the same observation in different clothes: if a set of cells must contain a set of digits, and the two sets are the same size, nothing else can have those digits.
A naked pair is two cells and two digits. A naked triple is three and three. An X-Wing is two lines and two cross-lines. A Swordfish is three and three. Once you see that they are one idea, the harder patterns stop feeling like separate things to memorise.
If you are earlier than this — still working out how the grid constrains itself at all — start with how to play Sudoku instead.
Frequently asked questions
What order should I learn Sudoku techniques in?
Scanning and hidden singles first — they solve most easy and medium boards on their own. Naked pairs next, because they are the first technique that eliminates candidates rather than placing digits. X-Wing and Swordfish come last; they only pay off once the simpler moves have run out.
Do I need advanced techniques to finish most Sudoku puzzles?
No. Easy and medium puzzles rarely need anything beyond scanning, hidden singles and naked pairs. X-Wing and Swordfish exist for hard and expert grids where every simpler line of reasoning is exhausted.
Are pencil marks cheating?
No. Writing candidates into cells is how essentially every strong solver works, and every technique past hidden singles is expressed in terms of candidates. The board on this site has a notes mode for exactly that reason.
Put them to work
Free 9×9 boards with pencil marks on the home page, a new puzzle daily, and live matches when you want the same techniques under time pressure.