Hidden Pair

If two digits can only go in the same two cells of a unit, every other candidate in those two cells can be removed.

Level: Intermediate · technique 7 of 7

When to use it

It is the naked pair seen from the digits' side. Other candidates crowd the two cells and hide the pair, which is what makes it harder to find.

Look for a digit that appears as a candidate in only two cells of a unit, then check whether a second digit appears in exactly the same two. Once you clear them, the two cells become a naked pair.

How to find it

1. Pick a row, column or box.
2. Find a digit that is a candidate in exactly two of its cells.
3. Check whether another digit is a candidate in exactly the same two cells.
4. If so, remove every other candidate from those two cells.

Worked example

On the board below, look at digits 3, 7 in column 6.

1. In column 6, digit 3 can only go in r3c6, r8c6, and so can 7.
2. The two digits have to share those two cells, so there is no room left in them for anything else.
3. Remove the other candidates from the two cells: 2 from r3c6, 4 from r8c6.

Common mistakes

• Each of the two digits must appear in those two cells only. If one also appears in a third cell, there is no hidden pair.
• The eliminations happen in the pair cells themselves, not in the rest of the unit - the opposite of a naked pair.