sudokUI › Sudoku techniques › Pattern Overlay
Pattern Overlay in sudoku
every one-digit pattern at once · how it fits
Every complete pattern for one digit X is listed: nine cells, one in each row, column and box, containing every placed X and otherwise only cells with candidate X.
Why it works
In the solution, the nine cells holding X form exactly one of these patterns. A candidate X that lies in no pattern is eliminated, and an empty cell that lies in every pattern must be X.
How to spot it
Pick a digit with several already placed and few candidates left, so that only a handful of complete patterns remain to be written out.
Worked example
Pattern Overlay: of the 6 complete placements possible for digit 9, none uses r9c1.
The solver uses this on the hardest puzzles. It has no pattern to spot, so it is not offered in practice.
In a puzzle's rating
Each time the solver needs Pattern Overlay, the puzzle's difficulty rating grows by 500. Its class is Extreme. It is needed in 1 in 740 puzzles that sudokUI generates. How the rating works.
More last resort
Exocet · Double Exocet · Forcing Chain · Digit Forcing Chain · Nishio Forcing Chain · Cell Forcing Chain · Unit Forcing Chain · Forcing Net · Brute Force