Naked Pairs are easily identified in a puzzle. For example:
In Row E, [E2] and [E8] have two identical candidates, which form a number pair {2, 3}. This makes candidates 2 and 3 be removed from the other cells in this row. Why? Because if [E2] = 2, then [E8] must be 3; Or if [E2] = 3, then [E8] must be 2. There is no other circumstance. Therefore, digits 2 and 3 must not be present at other cells in Row E. We should eliminate these two candidates from cells [E3], [E4] and [E5].
Similarly, we can find Naked Pairs in columns:
Number pair {6, 8} occurs only at cells [A3] and [H3] in Column 3, so we can safely remove all 3s from the candidates of all other cells in this column. In this case, candidates at [C3] and [F3] are updated.
The example of applying this technique in a block is as follows:
In Block at [G4], both [G5] and [I4] contain number pair {2, 4}. Similarly, we can remove these two candidates from the other cells in this block. In this example, totally four cells are affected which are [G4], [H4], [I5] and [I6].
Now we summarize the conditions to be satisfied in order to use this technique:
If two cells in a column, row or block contain an identical pair of candidates and only those two candidates , then no other cells in that column, row or block could be those values.
See Also:
