Unique rectangle: prevent a two-solution loop
A Type 1 unique rectangle has four cells in two rows, two columns, and two boxes. Three cells contain the same pair; the fourth contains that pair plus extras.
Key idea
If the fourth cell also took a pair digit, the rectangle could swap values and create two solutions. Its pair candidates must therefore be removed.
See the move, step by step
Inspect a 5/7 rectangle whose fourth corner also contains 9.
Step 1 of 4
Outline the four corners
Check two rows and two columns that cross exactly two boxes.
Step 2 of 4
Confirm three pure pair cells
Three corners contain only 5/7; R3C4 contains 5/7/9.
Step 3 of 4
Protect uniqueness at the extra corner
Remove 5 and 7 from R3C4. Otherwise all four corners could become a swappable 5/7 loop.
Step 4 of 4
Place the remaining value
R3C4 is left with 9, breaking the deadly rectangle.
Try it on today's puzzle
Use uniqueness techniques only on puzzles guaranteed to have one solution. Verify the four cells span exactly two boxes, not four.
Play today