Unique Rectangle Type 2
Two adjacent rectangle corners share one extra candidate. One must take it, so cells seeing both lose it.
What it is
Type 2 starts with four empty corners in exactly two rows, two columns and two boxes. The two floor corners contain only the same pair. Each adjacent roof corner contains that pair plus the same one extra digit. If neither roof took the extra, all four corners would reduce to the pair and create a second solution. The extra must therefore occupy one roof, and no other cell seeing both roofs can contain it.
How to spot it
Find the two exact-pair floor corners. The other two corners must be adjacent, share a row or column, and each have exactly the same single extra candidate. Inspect only cells that see both roofs; those are the safe targets for the shared extra digit.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 4: The starting position
- Pattern cells
- 4/8, 4/9, 8/8, 8/9
- Digits
- 5, 7, 9
- Eliminations
- 7/9
Press a marked square to point it out on the board above.
Why it matters
Type 2 turns a uniqueness condition into an ordinary candidate exclusion. The roof digit is not guessed: one roof must contain it to prevent the deadly rectangle, while Sudoku permits that digit only once in every shared unit.
Practise now
Solve a puzzle where this technique decides the game
Frequently asked questions
What makes this Type 2 rather than Type 1?
Type 1 has three exact pair corners and removes the pair from the remaining corner. Type 2 has two exact floor corners and two adjacent roofs with the same single extra digit; it removes that extra from cells seeing both roofs.
Must the two roof cells be adjacent?
Yes. Standard Type 2 needs two non-diagonal corners sharing a row or column. Their common extra is forced into one of those cells, which creates the eliminations.
Does this depend on the completed solution?
No. The hint reconstructs the rectangle, candidate sets, shared extra and every target from the visible grid only. The completed solution is never consulted.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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