Pointing Pairs, Triples and Claiming: Locked Candidates Explained
Pointing Pairs, Pointing Triples, and Claiming are three forms of the same core Sudoku concept: a candidate digit is 'locked' into a single row or column within a 3x3 box. This powerful restriction allows you to eliminate that digit from the rest of the line or the rest of the box. Collectively known as Locked Candidates or Box-Line Reduction, these are essential intermediate techniques that bridge the gap between basic singles and more complex strategies like [Naked Pairs](/techniques/naked-pairs). Mastering them is key to solving tougher puzzles efficiently.
The Core Idea: Locked Candidates
The Locked Candidates concept exploits the interaction between a 3x3 box and the rows or columns that pass through it. In standard Sudoku, a digit must appear exactly once in every row, column, and box. If, within one box, all possible positions for a specific digit lie in the same row or column, that digit is effectively 'claimed' by that line for that box. This creates a powerful constraint: the digit cannot appear elsewhere in that line (outside the box), or conversely, it cannot appear elsewhere in that box (outside the line). This duality gives rise to the two main patterns: Pointing and Claiming.
Pointing Pairs & Triples
A Pointing Pair (or Triple) occurs when all candidates for a specific digit within a 3x3 box are confined to a single row or column. Because the digit must go in that box, and its only options are in one line, the digit must ultimately occupy one of those cells. Therefore, you can eliminate that candidate from the rest of that row or column outside the box.
How to spot it: Scan each box for a digit. If every pencil mark for that digit falls within the same row or column of that box, you've found a Pointing Pair (two cells) or Pointing Triple (three cells). The elimination target is the rest of that row or column in the other two boxes.
Example: In a box, the digit 7 only appears as a candidate in two cells, both in the top row. Since the 7 for this box must be in its top row, the 7 cannot be in the top row of the other two boxes in that band. Eliminate candidate 7 from those cells.
- Focus your scan one box and one candidate at a time. This is easier than trying to process the whole grid.
- Pointing Triples are less common but follow the exact same logic. Don't be tricked by the number of cells; focus on the confinement to one line.
Claiming (or Box-Line Reduction)
Claiming is the reverse, or complementary, pattern to Pointing. Here, within a specific row or column, all candidates for a digit are confined to a single 3x3 box. Because the digit must appear somewhere in that row or column, and its only possible locations are inside one box, the digit is 'claimed' by that box for that line. You can therefore eliminate that candidate from all other cells within that box.
How to spot it: Scan each row and column for a digit. If every pencil mark for that digit in that line falls within the same 3x3 box, you've found a Claiming pattern. The elimination target is the other cells inside that box.
Example: In a row, the digit 5 only appears as a candidate in three cells, all of which are inside the same center box. Since the 5 for this row must be inside this center box, the 5 cannot be in any other cell of that center box. Eliminate candidate 5 from those other cells. This technique is also formally called Box-Line Reduction.
Why These Techniques Are Essential
Locked Candidates are often the first 'interaction' technique solvers learn. They create necessary eliminations that reveal Naked Singles or simplify other candidate lists, paving the way for more advanced patterns. Without spotting these, you might prematurely turn to guesswork or miss simpler logical steps. Our hint engine finds that mastering Pointing Pairs and Claiming significantly reduces the time players spend stuck on intermediate puzzles. These strategies work hand-in-hand with other pairing techniques; for instance, a Locked Candidates elimination might immediately create a Hidden Pair elsewhere on the grid.
Key Facts
- ▪Locked Candidates is a family of Sudoku techniques that includes Pointing Pairs, Pointing Triples, and Claiming.
- ▪A Pointing Pair occurs when a candidate appears only in one row or column within a 3x3 box, allowing elimination from the rest of that line.
- ▪The elimination rule for a Pointing Pair is: remove the candidate from the rest of the row or column outside its original box.
- ▪Claiming is the reverse: a candidate in a row/column is confined to one box, so it can be eliminated from the rest of that box.
- ▪Pointing Triples follow the same logic as Pointing Pairs but involve three cells in the box aligned in one row or column.
- ▪These techniques are also known as Box-Line Reduction, highlighting the interaction between a box and a line.
- ▪Locked Candidates are considered essential intermediate strategies, often required to solve puzzles rated Medium or harder.
- ▪Spotting these patterns requires scanning each box (for Pointing) and each row/column (for Claiming) candidate by candidate.
Frequently Asked Questions
What's the difference between Pointing and Claiming?
Pointing looks inside a box: if a candidate is locked to one line, eliminate it from the rest of that line. Claiming looks along a line: if a candidate is locked to one box, eliminate it from the rest of that box.
Is a Pointing Triple stronger than a Pointing Pair?
No, the logical strength is identical. Both confirm the candidate is locked to a specific line within the box. The number of candidate cells (2 or 3) doesn't change the elimination power.
Do I need pencil marks to find Locked Candidates?
Yes, these are candidate-based techniques. You must note possible digits in empty cells to see if they are confined to one row, column, or box.
How is this different from a Naked Pair?
A Naked Pair involves two cells in a unit sharing the same two candidates. Locked Candidates involves one candidate's positions being confined, creating cross-unit eliminations. They are different pattern types.
Which should I look for first, Pointing or Claiming?
Systematically scan boxes for Pointing Pairs/Triples, then scan rows and columns for Claiming. Both are equally important, and our solver engine checks for them simultaneously after basic singles.