The intuition
Locked candidates are a consequence of the box rule colliding with the line rule. The box must contain the digit somewhere. If every legal spot in the box happens to lie on a single shared row, then by the row rule the digit is also nailed to that row. The intersection — those two or three cells — owns the digit jointly, and everything else in the row outside the box is forbidden.
When you'll use it
Pointing Pairs appear in nearly every Hard puzzle and many Mediums. They are very often the move that breaks a stall after Naked Pairs have eliminated enough noise to expose the line-up.
How to spot a Pointing Pair
- Pick a 3×3 box and a digit.
- Mark every empty cell in the box where that digit is still a candidate.
- If those candidates all sit in a single row, remove the digit from that row outside the box.
- If they all sit in a single column instead, remove the digit from that column outside the box.
Why the Pointing Pair logic works
Sudoku requires every digit to appear exactly once per box. If the digit 5 must live in box 4, and its only candidate positions in box 4 all fall on row 5, then row 5's instance of digit 5 is going to come from box 4. Any other cell on row 5 that previously listed 5 as a candidate is wasting its time — the row's 5 is already accounted for inside box 4.
A worked Pointing Pair example
In box 1, suppose digit 5 can only go in R1C2 or R1C3. Both candidates sit on row 1. Therefore 5 must occupy row 1 somewhere inside box 1 — so 5 cannot appear anywhere else in row 1, i.e. in any of R1C4 … R1C9.
Walkthrough
Look at box 1 (rows 1–3, columns 1–3). Row 2 and row 3 of that box are already filled with digits other than 5. So 5 must go somewhere in row 1 of box 1.
Row 1's box-1 cells are R1C1, R1C2 and R1C3. But R7C1 = 5 kills R1C1 (same column). That leaves only R1C2 and R1C3 as candidates for the 5 in box 1 — and both are on row 1.
The 5 of box 1 is therefore committed to row 1 inside the box. The rest of row 1 (R1C4 through R1C9) cannot host the row's 5 — those candidates are eliminated even though their cells are otherwise empty.
This is also why Pointing Pair is sometimes called Locked Candidates Type 1 or Intersection Removal: the digit is locked at the intersection between a box and a line.
How to drill this technique
Open a Hard puzzle, pencil-mark exhaustively, and for each box look at digits 1 through 9 in order. For each digit ask 'are all its candidates here on one line?'. Do this for every box. You'll typically find one or two Pointing Pairs per board.
Common mistakes
- Applying the rule from a row inward. Pointing Pair points outward from the box, never into it.
- Confusing it with Box/Line Reduction (also called Claiming or Locked Candidates Type 2), which is the inverse: a line restricting a digit to one box, eliminating elsewhere in the box.
- Forgetting to check both directions — boxes can point along rows or along columns.
FAQ
Is Pointing Pair the same as Locked Candidates?
Pointing Pair (sometimes Pointing Triple when three cells line up) is one of two locked-candidate techniques. The other, Box/Line Reduction, goes in the opposite direction. Together they're called Locked Candidates Type 1 and Type 2.
Can a Pointing Pair eliminate candidates inside the same box?
No. Pointing Pair never eliminates inside its own box — only outside it, along the line on which the candidates lie.
How is Pointing Pair different from Naked Pair?
Naked Pair is about two cells with the same two candidates. Pointing Pair is about one digit whose candidates within a box happen to sit on one line. They look similar on the page but use different rules.
What is Box/Line Reduction in Sudoku?
Box/Line Reduction (also called Claiming, or Locked Candidates Type 2) is the inverse of Pointing Pair. Here a row or column restricts a digit to a single box — so the digit can be eliminated from the rest of that box. Pointing Pair points from a box outward; Box/Line Reduction points from a line inward.
Does Pointing Pair work with three cells too?
Yes — when all three of a digit's candidates in a box sit on the same row or column, it's called a Pointing Triple. The logic and the elimination work exactly the same way; you just have three locking cells instead of two.
What to learn next
Once Pointing Pair feels natural, move on to Box/Line Reduction — it builds directly on the candidate-tracking habits you've just developed. Or revisit the full Sudoku techniques library to see the full progression from beginner to advanced.