Pattern Overlay Method in Sudoku: A Practical Guide

Think in complete templates, not isolated candidates

Choose one symbol, such as the crescent in Shapedoku, and ignore the other eight for a moment. A complete template for that symbol contains nine cells: exactly one in every row, one in every column, and one in every 3 by 3 box. Every finished Sudoku contains one such template for each symbol. The Pattern Overlay Method, often shortened to POM, asks which crescent templates are still compatible with the clues and candidate Notes on the current board. Any template that uses a cell already blocked by a crescent in the same row, column, or box is discarded. The surviving templates represent every possible way the crescents could be arranged without breaking the rules. This changes the scale of the search. Instead of asking whether one candidate looks plausible, you ask whether it belongs to any complete legal arrangement. The method works identically with digits, but distinct shapes can make a single-symbol sweep easier to follow visually.

Use the surviving templates for eliminations and placements

Once the valid templates are listed, compare them cell by cell. If a crescent candidate appears in no surviving template, it cannot be part of the final solution and may be removed. Every possible complete arrangement avoids that cell, so the elimination is certain. The opposite result is even stronger. If every surviving template places the crescent in the same cell, that cell must contain the crescent. A small example might leave three legal crescent templates. Two pass through one candidate, while none uses another candidate in the same region. The unused candidate can be deleted immediately. If all three templates share a cell in a different box, that shared cell is solved. POM can therefore produce both candidate removals and direct placements. The reasoning does not guess which template is correct. It keeps all legal templates alive and uses only conclusions they share. That is why the method remains logical even though it considers several possible full arrangements at once.

Apply POM only when the pattern count is manageable

On an open board, one symbol may have far too many legal templates to track comfortably. POM becomes practical later, after clues and previous deductions have reduced the candidate map. Start with a shape that already has several confirmed placements or very few candidates in each box. Build one route at a time, making sure it visits every row, column, and box exactly once. Label the routes A, B, and C on paper, or use a separate sketch so temporary paths do not become permanent Notes. In an Extreme Shapedoku puzzle, smart cleanup helps keep the candidate map accurate, but the template comparison itself still requires patience. Stop if the number of routes grows confusing. A first free Hint may reveal that a simpler pair, chain, or fish was available. Pattern Overlay is best treated as a late tool for a sparse position, not as the first response to every difficult grid.

1. Choose one shape with relatively few remaining candidates.
2. Trace every complete route that places it once in each row, column, and box.
3. Discard any route that conflicts with a clue or confirmed placement.
4. Remove candidates that appear in none of the surviving routes.
5. Place the shape in any cell used by every surviving route.
6. Return to ordinary scanning after each result, because the board may now contain simpler deductions.