- A true dead end on a well-formed puzzle always has a logical solution — you are missing a technique
- The most common cause at Hard level: a missed Naked Pair or Locked Candidates pattern
- Verify your pencil marks before looking for advanced patterns
- Work through techniques in order of complexity — don't jump to advanced until basics are confirmed clean
- Taking a fresh look at the grid (even after a short break) often reveals the missed pattern
First: Confirm It Is a Real Dead End
Before declaring a dead end, check two things:
- Are all your pencil marks current? Every recent placement should have removed that digit from all its peers.
- Have you applied Hidden Singles to every unit? It is very easy to miss a Hidden Single in a unit with many candidates.
These two checks resolve the majority of apparent dead ends. Only if both checks pass should you look for more advanced patterns.
The Hard-Level Checklist
Work through this list in order — stop and apply the technique as soon as you find one:
| Check | What to Look For |
|---|---|
| 1 | Locked Candidates (digit confined to one row/column in a box) |
| 2 | Naked Pairs (two cells in a unit sharing exactly two candidates) |
| 3 | Hidden Pairs (two candidates confined to two cells in a unit) |
| 4 | Naked Triples |
| 5 | Hidden Triples |
| 6 | X-Wing (digit confined to two cells in each of two rows/columns) |
The Most Commonly Missed Pattern
At Hard level, Locked Candidates is by far the most commonly missed technique. Players who are comfortable with pairs often jump straight to looking for pairs and skip the box-to-line interaction check entirely. A single locked candidate elimination frequently triggers a Hidden Single, which triggers a Naked Single, which cascades through 5–10 more placements.
When the Puzzle Itself Seems Wrong
If you have worked through the full checklist and still cannot move, consider whether you may have made an earlier mistake. The symptoms: a cell with no candidates, or a unit where a digit cannot go anywhere. If you see either, trace back — do not continue forward. See our guide on recognizing when a puzzle is unsolvable for help distinguishing real errors from solving errors.