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Hidden pairs

Follow two digits to their only two cells.

A cell can have a long candidate list and still be tightly constrained. Instead of asking what can go in each cell, ask where each digit can go in the whole row, column or box.

When to use it

When two digits each appear as candidates in exactly the same two cells of a unit, those cells must contain those two digits. Remove every other candidate from those cells. This rule does not remove the pair’s digits from unrelated cells outside that unit.

How to find it

  1. 01

    Choose a unit and find a digit with exactly two possible positions.

  2. 02

    Find another digit whose only positions in that unit are those same two cells.

  3. 03

    Keep those two digits in both cells; delete their other candidates.

A worked position from the catalog

v1-expert-0002 · position before step 28

Sudoku position with the recorded logical candidatesRows and columns are numbered 1 to 9. Find the pattern before revealing the supporting cells. A complete text table follows the diagram.c1r1c2r2c3r3c4r4c5r5c6r6c7r7c8r8c9r9
Small digits are the candidates at this exact step, including earlier logical eliminations. No answer is filled in for you.
Read the position as a cell table
Each cell’s placed digit or complete remaining candidate list.
CellValue / candidates
r1c1Placed: 1
r1c2{4, 6, 7}
r1c3{4, 7}
r1c4Placed: 9
r1c5{4, 5, 6, 7}
r1c6{4, 5, 6}
r1c7Placed: 3
r1c8Placed: 2
r1c9Placed: 8
r2c1Placed: 5
r2c2{4, 9}
r2c3{3, 4, 8, 9}
r2c4Placed: 2
r2c5{3, 4, 8}
r2c6{4, 8}
r2c7Placed: 1
r2c8Placed: 7
r2c9Placed: 6
r3c1Placed: 2
r3c2{6, 7}
r3c3{3, 7, 8}
r3c4Placed: 1
r3c5{3, 6, 7, 8}
r3c6{6, 8}
r3c7Placed: 9
r3c8Placed: 5
r3c9Placed: 4
r4c1Placed: 9
r4c2{4, 5}
r4c3{4, 5}
r4c4{6, 8}
r4c5{6, 8}
r4c6Placed: 2
r4c7Placed: 7
r4c8Placed: 3
r4c9Placed: 1
r5c1Placed: 7
r5c2Placed: 8
r5c3Placed: 1
r5c4Placed: 5
r5c5Placed: 9
r5c6Placed: 3
r5c7Placed: 6
r5c8Placed: 4
r5c9Placed: 2
r6c1Placed: 3
r6c2Placed: 2
r6c3Placed: 6
r6c4Placed: 4
r6c5Placed: 1
r6c6Placed: 7
r6c7{5, 8}
r6c8{8, 9}
r6c9{5, 9}
r7c1{4, 6}
r7c2Placed: 3
r7c3{5, 9}
r7c4{6, 8}
r7c5{2, 4, 5, 6, 8}
r7c6Placed: 1
r7c7{2, 4, 5, 8}
r7c8{8, 9}
r7c9Placed: 7
r8c1{4, 6}
r8c2{5, 7, 9}
r8c3Placed: 2
r8c4{3, 7}
r8c5{4, 5, 6, 8}
r8c6{4, 5, 6, 8}
r8c7{4, 5, 8}
r8c8Placed: 1
r8c9{3, 5, 9}
r9c1Placed: 8
r9c2Placed: 1
r9c3{5, 7}
r9c4{3, 7}
r9c5{2, 4, 5}
r9c6Placed: 9
r9c7{2, 4, 5}
r9c8Placed: 6
r9c9{3, 5}

Look first. Reveal one layer at a time.

Find the supporting cells, explain the restriction, then inspect the candidates it removes. The original puzzle link starts from its givens; this panel preserves the intermediate logical state.

Play the original puzzle from its givens

When the rule does not apply

In row 4, digit 3 can go only in c2 or c7, but digit 6 can go in c2, c7 or c9. The two shared positions do not form a hidden pair because 6 is not confined to them. Removing other candidates from c2 and c7 could delete a correct answer.

Do these positions establish a hidden pair of 3 and 6 in r4c2 and r4c7?

Show why

No. Digit 6 has a third possible position at r4c9. Both digits must be confined to the same two cells in the chosen unit.

Try another position

This comes from a different puzzle. Use the condition, rather than the location you remember from the first example.

v1-expert-0003 · position before step 18

Sudoku position with the recorded logical candidatesRows and columns are numbered 1 to 9. Find the pattern before revealing the supporting cells. A complete text table follows the diagram.c1r1c2r2c3r3c4r4c5r5c6r6c7r7c8r8c9r9
Small digits are the candidates at this exact step, including earlier logical eliminations. No answer is filled in for you.
Read the position as a cell table
Each cell’s placed digit or complete remaining candidate list.
CellValue / candidates
r1c1{1, 5}
r1c2Placed: 7
r1c3Placed: 8
r1c4{1, 4, 6, 9}
r1c5{1, 4, 5}
r1c6Placed: 3
r1c7Placed: 2
r1c8{5, 6}
r1c9{5, 9}
r2c1{1, 3, 5}
r2c2Placed: 2
r2c3Placed: 9
r2c4{1, 6, 7}
r2c5Placed: 8
r2c6{1, 6, 7}
r2c7{3, 7}
r2c8{3, 5, 6}
r2c9Placed: 4
r3c1{3, 5}
r3c2Placed: 4
r3c3Placed: 6
r3c4{2, 7, 9}
r3c5{5, 7}
r3c6{2, 7, 9}
r3c7Placed: 1
r3c8{3, 5, 8}
r3c9{3, 5, 7, 8, 9}
r4c1Placed: 7
r4c2{1, 3, 5, 9}
r4c3{1, 5}
r4c4Placed: 8
r4c5{1, 3}
r4c6{1, 2, 9}
r4c7Placed: 6
r4c8Placed: 4
r4c9{1, 2, 3}
r5c1{6, 8}
r5c2{1, 3, 8}
r5c3Placed: 2
r5c4{1, 4, 6, 7}
r5c5{1, 3, 4, 7}
r5c6{1, 4, 6, 7}
r5c7Placed: 5
r5c8Placed: 9
r5c9{1, 3, 7, 8}
r6c1{6, 8, 9}
r6c2{1, 3, 8, 9}
r6c3Placed: 4
r6c4{1, 2, 6, 7, 9}
r6c5{1, 3, 7}
r6c6Placed: 5
r6c7{3, 7}
r6c8{1, 2, 3, 8}
r6c9{1, 2, 3, 7, 8}
r7c1{4, 8}
r7c2Placed: 6
r7c3Placed: 3
r7c4{1, 5, 7}
r7c5Placed: 9
r7c6{1, 7}
r7c7{4, 8}
r7c8{1, 2, 5}
r7c9{1, 2, 5}
r8c1Placed: 2
r8c2{1, 9}
r8c3Placed: 7
r8c4{1, 4, 5}
r8c5Placed: 6
r8c6Placed: 8
r8c7{4, 9}
r8c8{1, 3, 5}
r8c9{1, 3, 5}
r9c1{4, 8, 9}
r9c2{1, 5, 8, 9}
r9c3{1, 5}
r9c4Placed: 3
r9c5Placed: 2
r9c6{1, 4}
r9c7{4, 8, 9}
r9c8Placed: 7
r9c9Placed: 6

A new position. Your deduction.

Which action is justified by this pattern? Inspect the full candidate state before choosing. You can ask for a hint at any time.

Choose the justified action

Play the original puzzle from its givens

Make the distinction

After a hidden-pair elimination, the two cells become a naked pair. The starting evidence differs: a hidden pair is a restriction on where digits appear, not a short candidate list you can spot in isolation.

Compare with naked pairs

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