Avoidable rectangle: stop solved values from forming a deadly loop
An avoidable rectangle is the uniqueness counterpart found after some rectangle corners are solved. One remaining candidate would complete an interchangeable pattern.
Key idea
That candidate must be removed because it would allow the same rectangle values to swap into another solution.
See the move, step by step
Use a real solved-grid position to test the remaining rectangle corner.
Step 1 of 4
Locate two matching row-column pairs
Rows 5 and 7 contain the four corners of the potential rectangle.
Step 2 of 4
Compare solved corners and the open corner
The placed values establish the repeating pattern, while R5C7 still contains the dangerous candidate 3.
Step 3 of 4
Reject the interchangeable completion
If R5C7 were 3, the rectangle values could be exchanged without changing any outside constraint.
Step 4 of 4
Keep the unique completion
R5C7 is left with 2, so the rectangle can no longer generate an alternative solution.
Try it on today's puzzle
Only use this on a guaranteed unique puzzle. Carefully verify which corners are solved and which candidate would reproduce the deadly rectangle.
Play today