Hidden Pair

There is a hidden pair when, in a unit (row, column or box), two numbers can only go in the same two cells. Each one has to go in one of those cells, and no other note can stay in them. The other candidates are removed from the notes of those two cells.

It is the naked pair seen from the other side, just as the hidden single is the naked single seen from the other side. With the naked pair you look at the cells and see that they only have two numbers; with the hidden pair you look at the numbers and see that they can only go in the same two cells. It is called hidden because the two cells, seen on their own, don't give the pair away: they have other notes that hide the pattern.

An example

1. There are no singles, locked candidates or naked pairs in this grid. Row 5 is missing six numbers: the 1, 2, 3, 4, 6 and 7. These are the notes of the six empty cells.
2. Let's look for the 4: it only appears in two cells, in columns 2 and 9. The 6 also only appears in those two cells.
3. The 4 and the 6 of row 5 have to go in these two cells, one in each. The other candidates in those cells can be removed: the 3 goes from the cell in column 2, and the 2 goes from the cell in column 9.
4. Now the 3 of row 5 can only go in one cell: the one in column 8. It is a hidden single.

After the clean-up, the two cells are left with only the 4 and the 6: the hidden pair has become a naked pair. The two techniques describe the same fact. The hidden pair lets you recognise it sooner, while the cells still have other notes.

Why it is harder to see

A naked pair jumps out at you: two cells with the same two notes. The hidden pair doesn't: the cells may have three, four or five notes each. What reveals the pair is that two numbers only appear in the notes of those two cells, in the whole unit. You have to count, number by number, how many cells each one can go in.

How to search

• Number by number, one unit at a time. For each missing number, count how many cells it can still go in. If it is one, it is a hidden single. If it is two, remember which.
• Look for two numbers with the same two cells. They are the hidden pair. If only one of the numbers can go in those two cells, there is no pair.
• Check whether it removes anything. If the two cells already have only those two numbers, the hidden pair is already a naked pair: there are no more candidates to remove in those cells.
• The same logic applies to three or four numbers. When they can only go in the same three or four cells, they form hidden triples and quads, covered at Hard level.

Your turn

Find a hidden pair and cross out the other candidates in its two cells, leaving only the pair's two numbers. Tap each cell, choose the number and press «Check».