The rule is 1 to 9 once each in every row, column and 3×3 box. Below are the thirteen techniques the hint searches, easiest first, in the order set out by SudokuWiki. Press a name for a page that works it through with a picture and four practice puzzles.
| Technique | In one line | Difficulty |
|---|---|---|
| Naked Single | Only one digit can go in this cell | Easy · naked singles only |
| Hidden Single (Box) | Only one cell in this box can hold that digit | Medium · up to hidden singles |
| Hidden Single (Line) | Only one cell in this row or column can hold that digit | Medium · up to hidden singles |
| Naked Pair | Two cells hold the same two candidates → erase that pair from the rest of the unit | Hard · up to pairs |
| Hidden Pair | Two digits fit in only the same two cells → erase every other candidate from those cells | Hard · up to pairs |
| Pointing Pair | Inside a box the digit sits on one line only → erase it from that line outside the box | Very hard · up to intersections and triples |
| Box/Line Reduction | Along a line the digit sits in one box only → erase it from that box outside the line | Very hard · up to intersections and triples |
| Naked Triple | Three cells' candidates total three digits → erase those three from the rest of the unit | Very hard · up to intersections and triples |
| Hidden Triple | Three digits' homes total three cells → erase every other candidate from those cells | Very hard · up to intersections and triples |
| X-Wing | In two lines the digit sits on the same two columns → erase it from the rest of those columns | Expert · up to advanced techniques |
| Y-Wing (XY-Wing) | Three two-candidate cells form a loop → erase from any cell that sees both wings | Expert · up to advanced techniques |
| Simple Colouring | Paint the digit's either-or chain in two colours → erase the side that contradicts itself | Expert · up to advanced techniques |
| Swordfish | In three lines the digit's places fall inside the same three columns → erase it from the rest of those columns | Expert · up to advanced techniques |
Stuck? Find the technique from what you can see
Turn notes on and read down the table. The higher rows are easier to spot and come up more often.
| If you see this | This technique |
|---|---|
| an empty cell has only one candidate written in it | Naked Single |
| in one box, a digit has only one cell it can go in | Hidden Single (Box) |
| in one row or column, a digit has only one cell it can go in | Hidden Single (Line) |
| two cells in the same unit hold the same two candidates | Naked Pair |
| in one unit, two digits fit in only the same two cells | Hidden Pair |
| inside one box, a digit's candidates all sit on a single line | Pointing Pair |
| along one line, a digit's candidates are all inside a single box | Box/Line Reduction |
| three cells hold only three different digits between them | Naked Triple |
| three digits fit in only three cells between them | Hidden Triple |
| in two rows, a digit's candidate cells sit on the same two columns | X-Wing |
| three two-candidate cells link up, each sharing one digit with the next | Y-Wing (XY-Wing) |
| places where a digit has exactly two homes chain together | Simple Colouring |
| in three rows, a digit's candidate cells all fall within three columns | Swordfish |
From the fourth technique down, you cannot spot these without notes. The top three are visible without them, which is why they are what you use when a grid has just started.
The first three place digits, the rest erase candidates
The first three tell you which digit a cell takes. From the fourth on, nothing is placed — the candidates shrink instead. Hints remember what they erased, and once the candidates are thinner the first three find cells to fill again.
Auto notes
From the fourth technique on, you cannot see the pattern unless the candidates are written down. Turn on Auto notes in settings and every empty cell gets its candidates, recalculated after each move.
When thirteen are not enough
Every puzzle this game hands you is checked to finish within those thirteen techniques. A board you type into the solver yourself may need a harder one, and the hint will say so.