Sudoku techniques when stuck: what to check next
Updated
You stare at a quiet grid and want a formula that finishes it in five steps. There isn’t one. What follows is a check order. You try the next check only when the earlier ones have nothing to place or delete. This guide takes you as far as X-Wing. Expert puzzles may also need Swordfish or XY-Wing. Failing to finish with these six checks alone does not put a puzzle outside Calmtile’s four grades. That depends on the full grading ladder. The next step is still not a guess.
Notes can be on or off at any point in the list. They are not one of the steps. The play screen offers manual Notes, off by default. An automatic candidate list is a different tool, and this screen does not provide one. Using notes does not move the puzzle into another grade.
The order
| Step | What you look for | Page |
|---|---|---|
| 1 | A cell with one digit left | Naked single |
| 2 | A digit with one home in a box, a row, or a column | Hidden single |
| 3 | Two cells, or two digits, locked together | Pairs |
| 4 | A digit locked to one line in a box, or one box on a line | Pointing |
| 5 | Three digits locked to three cells | Triples, defined with the grades |
| 6 | One digit limited to the same two columns in two rows | X-Wing |
Use them in this order. A pointing deletion often wakes up a new single, so after any deletion you return to step 1 before you hunt a harder shape. X-Wing is last because the earlier checks finish a great many grids. I still sometimes skip ahead. The grid is patient. I am the one who then has to undo a deletion.
Step 5 has no article of its own. A triple is the three-digit form of a pair: three cells in a house whose candidates are drawn from the same three digits, or three digits that appear only in the same three cells. A puzzle is Hard when a triple is the highest technique the grading ladder uses to finish it. The definition is on the difficulty page.
Where the grades sit
The grade of a puzzle is the hardest step the ladder needed in order to finish it.
| Grade | Hardest step the puzzle needed |
|---|---|
| Easy | Naked single or hidden single |
| Medium | A pair, or pointing / box-line reduction |
| Hard | A triple |
| Expert | X-Wing, Swordfish, or XY-Wing |
Swordfish and XY-Wing appear in that Expert line, and in the name list below, only as names.
Handwritten notes never move a puzzle from one grade to another. A full candidate list would not either. The grade was fixed when the puzzle was built.
Names, and nothing more
Naked single, hidden single, naked pair, hidden pair, pointing, box-line, naked triple, hidden triple, X-Wing, Swordfish, XY-Wing.
Swordfish and XY-Wing have no article of their own.
When the order stops
Still stuck after the six checks? Expert puzzles may also need Swordfish or XY-Wing. It may help to leave the grid for a moment and come back with fresh eyes. You don’t need to fill a cell just because one digit looks promising. A guess is not the next step. A puzzle the full ladder cannot finish is not given one of the four grades.
The worked examples live on the earlier pages. Keep this list beside the grid. After a deletion, start again at the top. A new single may be waiting.
Next
If you want the grades, the names other people use, and why a count of givens is not the grade, they are on easy, medium, hard and expert Sudoku.