Naked Pairs
For all future methods we will assume that the candidates in a board are filled in correctly. You can
automatically calculate the candidates for each board from the settings menu.
The Naked Pairs method an extension of Naked Singles. In this method there are 2 cells with exactly 2 candidates in a
given row, column or square. Since the 2 values will be placed in one of the two cells, we know that
no other cells in the given house (row, column or square) will have these two values.
Naked Pairs - Example 1
Let us consider the following example. Looking at row E, cells E1 and E2
have only 2 candidates {1, 7}. We know that these two cells will be either 1 or 7.
Hence, in row E there cannot be other 1s or 7s and we can
eliminate them from E8 and E9.
To see why this method works, consider, for
example, what will happen if we put value 7 in cell E8. Placing a 7 there
will eliminate it as a candidate from both E1 and E2. Placing the remaining 1
in either of the cells will leave the other with no possibilities. The same is true for the candidates
{1, 7} in cell E9.
Naked Pairs - Example 2
As with most other simple methods, it is not necessary that we find a Naked Pair in a row. Using the same board, in a couple of steps we see a Naked Pair in the top-left square. The cells B1 and B2 have exactly two candidates {5, 6}. Hence, there cannot be another cell with value 6 in the same square and we can remove the one in cell A2.
