Hidden Pairs in Sudoku: Finding Locked Twins in Crowded Units

By Avi Israeli · Reviewed by Ninely Engine Logic Council · Updated November 2026

What is a Hidden Pair?

A Hidden Pair occurs when two digits appear as legal candidates in only two cells within a unit (row, column, or 3×3 box), even though those two cells also contain other candidate numbers.

Because those two digits cannot be placed anywhere else in that unit, they are strictly locked into those two cells. Therefore, every other candidate in those two cells can be discarded, turning the hidden pair into a clean Naked Pair!

Worked Example: Hidden Pair {4, 8} in Box 3 Locks Cells R2C7 & R2C8

Examine Box 3 (top-right 3×3 box, Rows 1–3, Columns 7–9). Box 3 contains fixed digits 1, 2, 3, 6 and has five empty cells: R1C7, R2C7, R2C8, R3C8, and R3C9.

  • Missing digits in Box 3: {4, 5, 7, 8, 9}.
  • Where can 4 go? Row 1, Row 3, and Column 9 are blocked by 4s elsewhere on the board. In Box 3, 4 can only go in R2C7 and R2C8!
  • Where can 8 go? Similarly, Row 1 and Row 3 are blocked by 8s. In Box 3, 8 can only go in R2C7 and R2C8!
  • Elimination: Both cells currently hold candidates {4, 5, 8, 9}. Because 4 and 8 are locked into these two spots, candidates 5 and 9 are eliminated from both cells, leaving a locked pair of {4, 8}!
Before: Crowded Candidates in R2C7 & R2C8
45 89
45 89
➔
After: Stripped to Locked Pair {4, 8}
4 8
4 8
8
4
6
3
1
2
7
2
4589
4589
6
1
8
4
3
4
1
7
3
9
6
8
5
7
1
9
9
2
1
6
8
4
6
3
2
2
9
5
7
3
6
1
8
2
4
Step 2 of 3: Digits 4 and 8 appear nowhere else in Box 3 (Hidden Pair)

⚠️ The Tempting Wrong Move: Placing 5 or 9 in R2C7

Because 5 and 9 appear across multiple empty cells in Box 3, a solver might prematurely place 5 in R2C7.

Why this fails: If R2C7 is filled with 5, Box 3 is left with only one remaining valid cell (R2C8) to place both 4 and 8. One of those two numbers will have nowhere to go, completely breaking the puzzle!

💡 Solving Tip: Count by Digit, Not by Cell

Naked pairs are easy to spot because you look for cells with only two pencil marks. Hidden pairs require the reverse mindset: look at the digits! For every unplaced number in a box or line, count how many cells can legally hold it. When two numbers share the exact same two candidate cells, you have found a Hidden Pair.

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