puzuru › Sudoku › Sudoku Strategies › Pointing Pair Also called locked candidates. Along with pairs, it is what Hard and Expert puzzles on puzuru may need once singles run out.
What is a pointing pair? Inside a 3x3 box, look at where one number can go. If all of those cells are in the same row (or the same column), the number must end up on that line inside this box.
So the number can't appear anywhere else on that line, outside the box. The pair "points" along the line and clears it.
1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 7 6 4 8 6 1 6 8 1 3 1 4 2 6 6 6 6 6 5 6 6 1 8 1 6 6 4 3 5 3 4 1 8 9 2 7 6 6 5 9 4 6 1 2 4 1 8 9 2 6 6 8 6 6 6 6 4 1 9
Example from a real puzuru puzzle. Rows and columns are numbered 1 to 9 from the top left. The example The diagram shows only the candidates for 6. In box 4, a 6 can only go in R4C1, R4C3. Both cells are in row 4.
Whatever happens, the 6 for box 4 will be in row 4. So no other cell in row 4 can be 6. Remove it from R4C5, R4C6.
How to spot it Work one number at a time, box by box. For each box, ask whether that number's candidates line up in a single row or column.
Two or three cells in a line both count. Three cells in a line is sometimes called a pointing triple.
The reverse: box/line reduction It also works the other way. If a number's candidates in a row are all inside one box, the number must go in that box on that row, so you can remove it from the rest of the box.
Many solvers call both directions locked candidates.
FAQ Is a pointing pair the same as locked candidates? Yes. Pointing pairs and box/line reduction are the two kinds of locked candidates.
Do I need notes for pointing pairs? It helps a lot. With notes on, you can see which cells of a box still hold each number.
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