Naked and Hidden Pairs

The first step in solving a Sudoku is eliminating impossible candidates, the figures that appear already in the same group (row, column, or block). If only one candidate remains in a cell, you can fill it in immediately (naked single). If a figure appears as a candidate in only one cell in a group, you can also fill it in (hidden single). If you can't solve the Sudoku with these methods, you can check for naked or hidden pairs, see the diagram below.

Naked pair

There is a naked pair if two cells in a group have the same two candidate figures. You can eliminate these candidate figures from all other cells in the same group.

In the diagram, 2 and 9 are the only candidates in both cell H7 and H9. If H7 would be 2, H9 would be 9, and the other way around. In both cases, 2 and 9 can not be a candidate in any other cell in the same group, in this case block 9. So, the 2 in cells G8 and G9 can be eliminated, the remaining candidates are 67 and 57.

Hidden pair

Hidden pairs are more difficult to spot. There is a hidden pair if there are two figures, that are candidate only in the same two cells within a group. You can eliminate all other candidates in those two cells.

In the diagram below, you see row A, the top row from the diagram above. The figures 6 and 8 can only be filled in in cells A5 and A6. If A5 is a 6, A6 must be 8, and the other way around. In both cases, you can eliminate the other candidates.

Hidden Pair

Now you can proceed, cell A1 is the only cell where you can fill in a 7 in row A, etc.