Claiming Pairs in Sudoku: Box-Line Reduction Explained

What a claiming pair means

Choose one symbol and inspect a single row. If every possible position for that symbol in the row lies inside the same 3 by 3 box, the row claims that symbol within the box. The symbol must appear somewhere among those row cells, so it cannot appear in any other cell of the same box. When exactly two positions remain, the pattern is commonly called a claiming pair. When three positions remain, it is a claiming triple. The logic is identical in a column. Suppose a row has only two possible cells for the lotus, and both cells sit in the center box. One of those cells must contain the lotus because every row needs one. Therefore, every other lotus candidate in the center box can be removed. Neither of the two row cells is solved, but together they reserve the box's lotus for that row. This technique is also called box-line reduction because a line restricts what can happen inside a box.

How claiming differs from pointing

Pointing and claiming use the same intersection from opposite directions. In a pointing pair, you begin inside a box. If a symbol's candidates in that box all lie on one row, you remove the symbol from the rest of that row outside the box. In a claiming pair, you begin with the row. If all candidates for the symbol in that row lie inside one box, you remove the symbol from the rest of that box outside the row. Remember the direction of the elimination. Pointing removes from the line. Claiming removes from the box. Mixing them up is one of the easiest ways to make an invalid candidate deletion. The pattern does not require the candidate cells to be adjacent, and a claiming triple is just as valid as a pair. What matters is complete confinement. If even one additional candidate for the symbol remains elsewhere in the row, the row no longer claims the symbol inside that box.

How to find claiming patterns consistently

Scan one shape at a time rather than checking random cells. In Shapedoku, select or mentally follow the heart, then inspect each row for two or three heart candidates that fall within one box. Repeat across columns before moving to the next shape. Claiming patterns often appear after a placement or candidate cleanup removes the last outside option from a line, so revisit affected rows and columns whenever a box changes. They are especially useful on Medium and Hard puzzles, where the board may stop offering direct singles but does not yet require long chains. Once you make a claiming elimination, check the box first. Removing one candidate may create a hidden single, a naked pair, or another locked candidate. Avoid hunting claiming pairs before your Notes are accurate. A missing candidate can create a false pattern, while a stale candidate can hide a real one. The technique is simple, but it depends on a complete map of every legal position for the chosen symbol.

1. Pick one shape and inspect its possible positions in a row.
2. Confirm that every position in the row falls inside one 3 by 3 box.
3. Remove that shape from other cells in the box but not from the claiming row.
4. Repeat the same search across columns.
5. Check triples as well as pairs, since confinement matters more than the count.
6. Rescan the affected box for singles and candidate subsets.