There is one rule — 1 to 9 once each in every row, column and 3×3 box. Below are the thirteen techniques this hint searches, easiest first. The order follows the technique ladder set out by SudokuWiki. Press a name to reach the page that works it through with an example — each one carries four puzzles that won't solve until you reach that technique.
| 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 |
When you're stuck — working backwards from what you see
The list above runs name → meaning. But someone stuck in front of a grid needs the opposite direction — finding the technique from what is in front of them right now. Turn notes on and read down the table below. The higher rows are easier to spot and come up more often.
| If you see this on the grid | This is the technique |
|---|---|
| some empty cell has only one candidate written in it | Naked Single |
| in one box, a digit has only one cell it could go in | Hidden Single (Box) |
| in one row or column, a digit has only one cell it could go in | Hidden Single (Line) |
| two cells in the same unit carry the identical 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 crowd onto a single line | Pointing Pair |
| along one line, a digit's candidates survive in only one box's share | Box/Line Reduction |
| three cells' candidates come to only three distinct digits between them | Naked Triple |
| three digits' possible cells come to only three 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 one after another | Simple Colouring |
| in three rows, a digit's candidate cells all fall within three columns | Swordfish |
If you can't see anything from the fourth row down, notes are probably off. From there on, the counting is impossible by eye unless the candidates are written down. The top three, by contrast, are visible without notes — those are the ones you use when a grid has just started.
The first three place digits; the rest erase candidates
The first three tell you “this cell is this digit”. From the fourth on, nothing gets placed — they only shrink the candidates. It looks like no progress, but it isn't: the candidates have to shrink before the first two catch again.
What you erased, the next hint remembers
A technique that erases candidates almost never achieves anything on its own. The road opens when the next technique stands on what the last one erased. So this game remembers what a hint erased and computes the next hint on top of it.
Use notes
From the fourth technique on, nothing is visible unless the candidates are written down. Turn on Auto notes and every empty cell gets its possible digits, recomputed after each move. That is where sudoku actually begins.
When even these run out
Some puzzles won't give up the next move even to all thirteen. When that happens this game does not invent a reason. It says plainly that a harder technique is needed from here, and gives you one cell. That seemed better than dressing up something plausible.