sudokUI › Sudoku techniques › Extended Rectangle
Extended Rectangle in sudoku
Six unsolved cells where two rows cross three columns, or two columns cross three rows, in exactly three boxes, holding three digits plus one cell's extras or one extra digit.
Why it works
Without extras, each box's two cells could swap values, giving a second solution. Assuming the puzzle has exactly one solution, the cell with extras loses the three digits, or the extra digit is removed from outside cells seeing every pattern cell holding it.
How to spot it
Look for three digits recurring in two rows or columns that run through the same three boxes, one pair of cells per box.
Worked example
Extended Rectangle Type 1: r8c1, r9c1, r8c5, r9c5, r8c9, r9c9 would form a deadly 249 loop; r9c5 must take one of its extra candidates.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Extended Rectangle 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 Extended Rectangle, the puzzle's difficulty rating grows by 110. Its class is Hard. It is needed in 1 in 690 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 · Avoidable Rectangle Type 2