sudokUI › Sudoku techniques › Avoidable Rectangle Type 2
Avoidable Rectangle Type 2 in sudoku
Unique Rectangle using cells you solved · how it fits
A rectangle in exactly two boxes: two corners in one row or column are solved, not givens, and each unsolved corner holds only Z and the digit diagonally opposite it.
Why it works
If neither unsolved corner were Z, the rectangle would hold two digits that could swap without touching a given, making a second solution. Assuming the puzzle has exactly one solution, one is Z, so Z is removed from every other cell that sees both.
How to spot it
Look for two of your own placements in one row or column of a rectangle whose remaining corners are bivalue cells sharing one candidate.
Worked example
Avoidable Rectangle Type 2: to avoid an interchangeable rectangle with the solved cells r6c3, r9c3, one of r6c2/r9c2 must be 3, so 3 is removed from cells seeing both. So 3 is removed from r7c2.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Avoidable Rectangle Type 2 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 2, the puzzle's difficulty rating grows by 100. Its class is Hard. It is needed in 1 in 28,000 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 1 · Extended Rectangle