Every technique, on a board that actually has one
Most guides draw a tidy diagram. These use positions pulled from real games — every number can be checked against the candidates.
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Beginner
Naked Single
One empty cell, one surviving candidate. It is the simplest move in sudoku, and the one most often confused with its cousin.
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Beginner
Hidden Single
The cell still looks undecided — several digits could go there. But for one of those digits, this is the only place left in the entire unit.
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Beginner
Candidate Notation
Every technique on this site points at a cell by name. If the name is not obvious, none of the reasoning that follows will be either.
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Intermediate
Naked Pair
When two cells in the same unit both hold exactly the same two digits, those digits are used up — and the rest of the unit has to do without them.
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Intermediate
Hidden Pair
When 4 and 6 can only live in the same two cells, those cells are theirs — and everything else pencilled into them is noise.
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Intermediate
Pointing
Every home for the digit inside this box sits in the same row. So the box has claimed that row's copy — and the rest of the row has to look elsewhere.
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Intermediate
Claiming
Every home for the digit in this row sits inside one box. The row will supply that box's copy — so the box cannot get it anywhere else.
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Intermediate
Scanning Workflow
Most stalled boards are not hard. They are being scanned in the wrong order, so the cheap move is hidden behind the expensive one.
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Intermediate
Pencil Marking
Candidates are not free. They cost pencil time and visual noise, and on most boards you need far fewer of them than the grid suggests.
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Advanced
Naked Triple
The three lists do not have to match. They only have to add up to three digits between them, no more.
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Advanced
Hidden Triple
The three cells may hold six different candidates between them. It does not matter — three digits have nowhere else to go, so those three cells are taken.
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Advanced
X-Wing
Four cells in a rectangle, one digit, and a guarantee strong enough to cross out candidates in two whole lines.
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Advanced
XY-Wing
The pivot cannot tell you which pincer fires. It only guarantees that one of them does, and that is enough to clear a cell.
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Advanced
XYZ-Wing
Three candidates in the pivot instead of two. The logic barely changes; the set of cells you may eliminate from shrinks sharply.
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Advanced
Skyscraper
The two lines share a base column. Only one of the base cells can hold the digit, which forces the other line's tower to take it.
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Advanced
Unique Rectangle
This one does not come from the rules of sudoku. It comes from the promise that the puzzle has exactly one answer.
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Advanced
Swordfish
Everything you know about X-Wing survives the upgrade. The only change is that three lines stand in for two.
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Expert
Jellyfish
The logic is unchanged from X-Wing and Swordfish. What changes is the cost of finding it — and whether that cost is worth paying.
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Expert
Forcing Chain
Every other technique looks for a pattern. This one just assumes a value and follows the consequences until the board contradicts itself.
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Expert
Stuck Board Checklist
Nearly every stuck board is a bad candidate, a missed sweep, or a technique applied in the wrong direction. This is the order to check them in.
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