sudokUI › Sudoku techniques › Avoidable Rectangle Type 1
Avoidable Rectangle Type 1 in sudoku
Unique Rectangle using cells you solved · how it fits
A rectangle in exactly two boxes with three solved corners, none of them givens: the two seeing the unsolved corner are both Y, and the one opposite it is X.
Why it works
If the unsolved corner were X, the four cells would hold only X and Y, and swapping them would give a second solution without changing any given. Assuming the puzzle has exactly one solution, X is removed from that corner.
How to spot it
Keep givens and your own placements visually distinct, then look for three of your placements on a rectangle using only two digits.
Worked example
Avoidable Rectangle Type 1: setting r2c9 to 2 would complete an interchangeable rectangle of solved (non-given) cells, contradicting the puzzle's unique solution.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Avoidable Rectangle Type 1 and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Avoidable Rectangle Type 1, the puzzle's difficulty rating grows by 100. Its class is Hard. It is needed in 1 in 2,000 puzzles that sudokUI generates. How the rating works.
More uniqueness
BUG+1 · Unique Rectangle Type 1 · Unique Rectangle Type 2 · Unique Rectangle Type 3 · Unique Rectangle Type 4 · Unique Rectangle Type 5 · Unique Rectangle Type 6 · Hidden Rectangle · Avoidable Rectangle Type 2 · Extended Rectangle