Pointing pairs in Sudoku: connect a box to a line
Choose one digit inside a 3×3 box and check all nine cells. If every remaining place lies on the same row or column, the box will supply that line’s digit from inside the overlap.
Definition
A pointing pair occurs when every possible place for one digit inside a 3×3 box lies on one row or column; that digit can be removed from the rest of the same line outside the box.
When to use it and what to verify
When to look for it
Map one candidate through all nine cells of a 3×3 box, especially when only two or three possible positions remain.
Required condition
Every remaining position for the digit inside the box must share one row or one column.
Exact elimination boundary
Remove the digit only from the shared line outside the box. Keep all candidate positions inside the box.
The box keeps candidate 5 on row 1.
- 1. Check every 5 in the boxIn the top-left box, candidate 5 appears only at R1C1 and R1C2.
- 2. Notice the shared lineBoth cells are on row 1. Whichever cell receives 5, the box places row 1’s 5 inside the box.
- 3. Clear only the extensionRemove candidate 5 from R1C4 and R1C6. Keep both 5s inside the box.
The top-left box must place 5 at R1C1 or R1C2. Therefore candidate 5 is impossible at R1C4 and R1C6.
A reference verified by Doku.su
Doku.su checks every placement or elimination in this example with its deterministic Sudoku engine. Illustrative candidates are labelled when they do not represent a complete puzzle state.
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