A Sudoku Solving Checklist to Run Whenever You Get Stuck

Why a Sudoku solving checklist helps

The value of a Sudoku solving checklist is that it replaces panic with process. When a puzzle stalls, the temptation is to stare at one region and hope, or worse, to guess. A checklist forces you to sweep the whole grid in a sensible order, cheapest and most common techniques first, harder ones only when the easy scans come up empty. This does two things: it makes sure you never miss a simple move because you jumped ahead, and it tells you honestly when you have earned the right to try something advanced. Run the list from the top each time you are stuck, and stop the moment any step produces a placement or an elimination, because that change may reopen everything above it.

The core checklist, in order

1. Naked singles: any cell with only one candidate left, place it.
2. Hidden singles: any digit with only one home in a row, column or box.
3. Naked pairs and triples: locked candidate sets that trim their house.
4. Hidden pairs and triples: digit groups confined to the same few cells.
5. Pointing pairs: a digit stuck to one line inside a box clears that line.
6. Claiming: a digit stuck to one box inside a line clears the rest of the box.
7. X-Wing and skyscraper: your first single-digit eliminations.
8. Wings: XY-Wing, XYZ-Wing and friends built on bi-value cells.
9. Chains: colouring, X-chains and XY-chains when all else is empty.

Re-scan after every change

The most important rule in the whole routine is to restart the checklist after any successful step. A single placement can create a new naked single three cells away, or a pointing pair can expose a hidden single you could not see a moment ago. Solvers waste huge amounts of effort by finding one move, then continuing to hunt for advanced patterns while an easy single sits waiting. Discipline yourself: found something, go back to the top. Only when a full pass from naked singles down through the intersections produces nothing at all have you truly reached the point where fish, wings and chains are needed.

Housekeeping that prevents dead ends

Two habits keep the checklist trustworthy. First, maintain an accurate candidate map; most stuck moments on Hard puzzles are really bookkeeping errors where a candidate should have been erased earlier. Use full Notes and, if your app offers it, smart cleanup that removes candidates a placement disproves. Second, when you apply an elimination technique, act on its target cells immediately rather than admiring the pattern, then re-scan. If you have run the entire list twice and still find nothing, recheck your map for a mark you forgot to remove, try each digit separately for a single-digit pattern, or simply take a short break, since fresh eyes catch missed singles surprisingly often.

A good Sudoku solving checklist is portable, working on paper, on a printout, or on screen, and it gets faster with use until the early steps take only seconds. Shapedoku is a comfortable place to build the habit, since its Notes view with smart cleanup handles the housekeeping for you and the first free hint can explain which step you missed. If you like solving away from a screen, shapedoku.com offers printable puzzles and a free solver to check your work. Keep the list, run it from the top, and stuck will start to feel temporary rather than final.