Sudoku Techniques, in the Order You Need Them

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Published April 29, 2026 · Updated October 20, 2026

Our Sudoku Helper knows eighteen techniques, and its hint button always plays the simplest one that works. We ran it over 800 boards from our own levels and over all 5,000 puzzles behind Evil Sudoku, and counted the techniques each puzzle could not be finished without. The result is lopsided. Our Easy to Expert levels never need more than the first five; the other thirteen are for Evil. And five further techniques we measured alongside them mostly take moves the eighteen would have made anyway.

This page is the map: every technique we have a guide for, in that order, with a line on what each one looks for and a link to the guide that teaches it. For the vocabulary itself (candidate, house, strong link) see sudoku terms; for the routine of a first puzzle, sudoku strategies for beginners.

What order should you learn sudoku techniques in?

The order our helper tries them in. It runs from patterns you can see at a glance to ones you have to search for, and each row is tried only when every row above it has found nothing.

The Sudoku Helper’s eighteen techniques in its own order. First needed on: the easiest of our levels where the technique was the helper’s simplest move on at least one of 200 boards. Last column: puzzles of the 5,000 in our Evil bank on which it was the simplest move at least once. Measured October 2026.

Two things the table does not show. Position is not frequency: almost locked sets come last, yet more Evil puzzles need one than need anything else past naked pairs, while the Jellyfish, eleventh, is needed by seven. And the order decides which technique gets the credit when two would do: the box/line reduction guide swaps it with pointing pairs and the two counts swap with them, because at 171 of the 223 pointing moves on our Expert boards a claim would have struck exactly the same candidates.

So learn the list from the top. Stop after the fifth row until you play Evil puzzles: nothing below it will ever be needed on a lower level of ours.

Which techniques does each difficulty level need?

We dealt 200 boards per level from the generator behind our games and walked each one with the helper:

• Easy and Medium: singles. Naked singles alone finish 192 of 200 Easy boards and 94 of 200 Medium; hidden singles do the rest.
• Hard: singles, then locked candidates. Singles finish 177 boards; the other 23 needed a pointing pair or a box/line reduction, and nothing else.
• Expert: add naked pairs. 68 boards needed more than singles, 17 of them a naked pair.
• Evil: the other thirteen. None of the 5,000 falls to the first five rows. The eighteen finish 3,655; the other 1,345 need something past the list.
• Extreme: past the list. On 4,980 of its 5,000 puzzles none of the eighteen can make a single move.

That is by design. Our generator takes a digit away only while the level’s own techniques can still finish the board, so no board can need more than its level allows. How each level is built, givens included, is on sudoku difficulty levels.

How do you find the basic techniques?

Singles are the only techniques that place a digit; everything after them strikes candidates until a single appears. A naked single is read from one square: its row, column and box already hold the eight other digits. A hidden single is read from one house: the digit has nowhere else to go. Scanning and cross-hatching are not extra techniques but ways to find hidden singles without writing anything: pick a digit, follow its copies along their rows and columns, and look for a box with one square left uncovered. A full house, the last empty square of a row, column or box, is the easiest naked single there is.

Locked candidates can still be read from the digits alone, as their guides show, but from the pairs on you need pencil marks. Every later technique is a rule for striking marks, and a missing one is the usual way to build a false pattern. If a move ever contradicts the solution, check the marks before the technique; sudoku mistakes shows how.

What comes after singles?

Two families, both confined to one or two houses.

• Locked candidates. Pointing pairs and box/line reduction are the two directions of one idea, where a box and a line cross, and the locked candidates guide shows both on one Hard board. A pointing triple works the same way as a pair.
• Subsets. Naked pairs and triples are read from squares, hidden pairs and triples from digits, and in any house each naked set has a hidden mirror that strikes exactly the same marks. Hidden pairs matter far more than their place suggests: on 1,530 of the 5,000 Evil puzzles, the first move beyond naked pairs is a hidden pair, more often than anything else. Quads exist too; the helper leaves them out, and the hidden pairs guide shows that costs almost nothing.

What are fish and wings?

A fish works on one digit. The X-Wing takes two rows where the digit has two places each, in matching columns, and clears it from the rest of those columns; Swordfish and Jellyfish do it with three lines and four. Give an X-Wing or Swordfish an extra place, the fin, and you have a finned X-Wing or finned Swordfish, which can clear only squares in the fin’s box. Fish thin out fast: of the 5,000 Evil puzzles, 644 need an X-Wing, 122 a Swordfish and 7 a Jellyfish. The Jellyfish guide shows why: every fish has a twin on the other lines, so a big one is nearly always a small one seen from the other side.

Wings join a few squares with two candidates each so that one of two digits must land where it strikes something. The XY-Wing (also called a Y-Wing) is the template, and its guide relates the others to it. An XYZ-Wing can clear at most two squares. A W-Wing can’t take a strong link that ends on one of its own two squares, and its guide shows the false W-Wing that results. On Evil, wings are the workhorses: 1,769 puzzles need a W-Wing and 1,743 an XY-Wing.

What are chains and colouring, and is a chain just guessing?

A chain follows forced consequences from square to square. The Skyscraper is the shortest that the helper knows, two strong links on one digit, and it can only clear squares where its two loose ends share a band or stack. Simple colouring joins every strong link a digit has. The rest are in our chain guides:

• Remote pairs: a run of squares that all hold the same two digits. Something simpler is nearly always there first, and once naked pairs are done simple colouring finds every one: only 2 of the 5,000 Evil puzzles needed one.
• 3D Medusa: colouring carried across digits through squares with two candidates, with six rules.
• X-cycles and X-chains: one digit, strong and weak links in turn. The 2-String Kite and the Turbot Fish are short ones.
• XY-chains: two-candidate squares, each sharing a digit with the next. An XY-Wing is the three-square case.
• Forcing chains: follow every option of a square or a digit and keep what they all agree on. Its guide covers Nishio too.

Chains are where the eighteen hand over. They finish 332 of the first 500 Evil puzzles. Adding BUG finishes 1 more, X-chains 14 more, XY-chains another 30, and forcing chains all the rest: 500 of 500. Our Extreme puzzles are another matter. There even forcing chains find nothing to start from: those puzzles need chains nested inside chains, and the forcing chains guide takes up whether that still counts as logic.

Which techniques assume the puzzle has one solution?

The unique rectangle and BUG. A unique rectangle is four squares in two boxes where two digits could swap places, giving a second solution, so the candidate that would allow it can go. BUG+1 does the same for a whole board: when every open square but one is down to two candidates, the odd square’s extra digit is its answer. Both are sound on a puzzle with one solution, and every puzzle we deal has exactly one. On a puzzle typed in from elsewhere that has two, a rectangle can strike both solutions, which is why our helper now counts a typed-in puzzle’s solutions and offers the rectangle only when there is exactly one. They are rare in any case: 195 of the 5,000 Evil puzzles need a rectangle.

What are almost locked sets?

An almost locked set is N squares of one house holding N+1 digits between them. Two of them that share a digit able to sit in only one of them give the ALS-XZ rule, the last of the helper’s eighteen and the one Evil leans on most: 2,761 of the 5,000 puzzles need it, and it is the hardest step on 1,668 of the 3,655 the eighteen finish. Sue de Coq is two almost locked sets around one box-and-line crossing. Several simpler patterns are ALS-XZ moves in disguise; the ALS guide lists them and goes on to ALS chains and Death Blossom.

Do you need techniques our helper doesn’t use?

Rarely. Our measurement runs five more alongside the eighteen: the two finned fish, remote pairs, 3D Medusa and Sue de Coq, each placed in the list where its guide explains. On the first 500 Evil puzzles they made 782 moves between them. At only 25 of those did the eighteen have nothing to offer.

The opening 500 of the Evil bank; the bank is kept in score order, which makes them tougher than the rest. Our measuring solver tries all 23 techniques simplest first; the last column counts moves made where none of the helper’s eighteen applied. Measured October 2026.

Learn them for speed or for the pleasure of the pattern, not as rescues. Other solvers sort techniques differently: SudokuWiki’s solver, for one, files Exocet, Death Blossom and the forcing chains under “Extreme Strategies”, and lists Remote Pairs, Multi-Colouring and Empty Rectangles as “Deprecated Strategies”. We have no guides to Exocet, Empty Rectangles, the WXYZ-Wing or pattern overlay. Our measurement has no detectors for them, so we can’t show them on our own boards or say how often our puzzles would use them.

What about killer sudoku techniques?

Killer adds arithmetic, and four guides cover it: the rules of killer sudoku, a combinations chart for every cage size, the rule of 45, and killer sudoku tips, which gives our killer solver’s order: cage arithmetic, singles, confined digits, pairs and triples, then the rule of 45. When the first two run dry, the next move is nearly always a confined digit, on 248 of 252 Expert boards that got that far. Easy boards on our Killer Sudoku page never need the rule of 45; Expert needs it on 124 boards in 300.

Stuck? How to find the next technique

Then practise the one you missed. The Sudoku Trainer has ten real positions for each of the eighteen, 180 in all, and in 171 the technique is the simplest move the board offers. If the helper names nothing at all, the board is past the eighteen: try the chain guides above, from X-chains to forcing chains.