sudokUI › Sudoku techniques › Locked Pair
Locked Pair in sudoku
Naked Pair clearing two units · how it fits
Two cells in the same box and the same row or column are both bivalue cells with the same two candidates, X and Y.
Why it works
Those two cells must take X and Y in some order, so no other cell that sees both can hold either digit. X and Y are eliminated from every other cell of the box and of the shared row or column.
How to spot it
If two bivalue cells in a box hold the same candidates and share a row or column, look for those digits elsewhere in both units.
Worked example
Locked Pair: cells r1c1, r2c1 contain only 3/7, removing those digits from the cells they all see. So 7 is removed from r3c1, r5c1, r6c1, r2c3, r3c3.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Locked Pair 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 Locked Pair, the puzzle's difficulty rating grows by 40. Its class is Medium. It is needed in 1 in 12 puzzles that sudokUI generates. How the rating works.
More intersections
Locked Triple · Locked Candidates (Pointing) · Locked Candidates (Claiming)