Pointing Pairs and Triples in Sudoku
A Pointing Pair or Triple is a Sudoku technique where all candidates for a digit within a single 3x3 box are aligned in one row or column. This alignment 'locks' the digit into that line within the box, allowing you to eliminate that candidate from the rest of the row or column on the Sudoku grid. Also known as Locked Candidates, this is a foundational intermediate strategy that builds on basic scanning and opens the door to more complex logic. Mastering it is often the key to progressing from easy to medium-difficulty puzzles.
What Are Locked Candidates?
In Sudoku, a 'locked candidate' is a digit whose possible positions in a 3x3 box are restricted to just one row or one column inside that box. This restriction creates a powerful logical consequence: if the digit must go in that specific line inside the box, then it cannot appear elsewhere in that same line outside the box. The pattern is called a Pointing Pair when two cells are involved, and a Pointing Triple when three cells form the line. This technique is sometimes grouped under the broader strategy of Box-Line Reduction.
How to Find and Use Pointing Pairs
The process for finding a Pointing Pair is systematic. First, pick a 3x3 box and a candidate digit. Scan the box and note all the empty cells where that digit could possibly go. If all those possible cells lie in the same row (or the same column) within that box, you have found a Pointing Pair or Triple.
Once identified, the logic is direct. Because the digit must be placed in one of those aligned cells inside the box, you can erase that candidate digit from every other cell in that same row or column that lies outside the box. This elimination often directly creates a Naked Single or a Hidden Single elsewhere.
- Focus on one box at a time to avoid confusion.
- Look for digits that appear as candidates in only 2 or 3 cells within a box.
- After an elimination, always re-scan the affected row, column, and box for new singles.
The Reverse Logic: Claiming (Box-Line Reduction)
This technique also works in reverse, in a pattern called Claiming. Here, you start with a row or column instead of a box. If all the possible positions for a candidate digit within a row are confined to a single 3x3 box, that digit is 'claimed' by that box. You can then eliminate that candidate from all other cells within that box. Claiming and Pointing are two sides of the same Locked Candidates coin, both enforcing the rule that a digit must appear once in every row, column, and box.
Why This Technique is So Valuable
Pointing Pairs and Triples are crucial because they act as a bridge between simple techniques and more advanced ones. They don't require complex pattern recognition like Naked Pairs or Hidden Pairs, but they create the clean board state needed to find those patterns. Our hint engine data shows that puzzles rated 'medium' difficulty almost always require at least one application of Locked Candidates to be solved. It's a deductive, rather than observational, step that trains you to think about candidate placement across unit boundaries.
Key Facts
- ▪A Pointing Pair occurs when all candidates for a digit in a 3x3 box are in two cells that share the same row or column.
- ▪A Pointing Triple is the same pattern but with three aligned cells containing the candidate.
- ▪The logic allows you to eliminate that candidate from the rest of the shared row or column outside the box.
- ▪This technique is also known as Locked Candidates Type 1 or Direct Pointing.
- ▪The reverse pattern is called Claiming (Locked Candidates Type 2), where a digit in a row/column is locked to one box.
- ▪Pointing and Claiming are both forms of Box-Line Reduction, a core intermediate Sudoku strategy.
- ▪These techniques are often essential for solving medium-difficulty puzzles and creating new Naked Singles.
- ▪They enforce the basic Sudoku rule that a digit must appear once in every row, column, and 3x3 box.
- ▪Unlike Naked Pairs, this technique focuses on the confinement of a single candidate digit across units.
Frequently Asked Questions
What's the difference between a Pointing Pair and a Naked Pair?
A Naked Pair involves two digits shared by two cells in a unit. A Pointing Pair involves one digit whose positions are locked to one row/column within a box, allowing eliminations in a different unit.
Can a Pointing Triple have the cells in a 'L' shape?
No. For a Pointing Triple, all three cells containing the candidate must be perfectly aligned in one row OR one column within the box. An 'L' or other shape does not lock the candidate to a single line.
What should I do after making a Pointing Pair elimination?
Always re-check the row, column, and box where you made the elimination. Removing that candidate often reveals a direct placement (a Single) or creates new Hidden Pairs.
Is there an opposite technique to Pointing Pairs?
Yes. The reverse is called Claiming (or Locked Candidates Type 2). If a digit in a row/column is confined to one box, eliminate it from the rest of that box.
How do I practice finding Pointing Pairs?
Use the hint feature on SudokuHint.com. On medium puzzles, when stuck, activate hints. It will often point out a Pointing Pair you missed, helping train your eye for the pattern.