TECHNIQUE · INTERMEDIATE
Hidden Pair — two digits lock two cells
When two digits in a unit appear as candidates only in the same two cells, those cells are jointly locked. Every other candidate on those two cells can be removed, even if they look crowded.
The logic
Suppose digits a and b each appear as candidates only in cells X and Y of one unit. The two cells must hold a and b between them, so any other candidate written on X or Y can be eliminated.
How to find them
Naked Pairs are spotted by counting candidates per cell. Hidden Pairs hide behind extra candidates, so you scan by digit instead. List the candidate positions for each unplaced digit in a unit and look for two digits that share the same two cells.
- Pick a row, column, or box and list candidate positions per unplaced digit.
- Find two digits whose candidate cells are exactly the same two positions.
- Remove every other candidate from those two cells.
Naked vs. Hidden Pair
Naked Pair looks at cells (two cells with exactly two matching candidates). Hidden Pair looks at digits (two digits confined to the same two cells). Knowing both views unblocks stalled boards much faster.
Practice order
- Full candidatesWrite every possible digit in every empty cell.
- List by digitWithin one unit, list candidate cells per unplaced digit.
- Match digit pairFind two digits that share the same two cells.
- Clear extrasRemove every other candidate from those two cells.
Walk through a Hidden Pair
Step 1 of 4
Read row 1. Cells 5–9 hold the digits 5, 6, 7, 8, 9. Cells (row 1, col 3) and (row 1, col 4) are already pruned to {3, 4}, while (row 1, col 1) and (row 1, col 2) still hold {1, 2, 3, 4}.