Naked Pairs

A Naked Pair (also known as a Conjugate Pair) is a set of two candidate numbers sited in two cells that belong to at least one unit in common. That is they reside in the same row, box or column.

It is clear that the solution will contain those values in those two cells (we just don’t know which is which at this stage) and all other candidates with those numbers can be removed from whatever unit(s) they have in common.

In Figure 2 we have a Naked Pair sited in box 7. It is also on row H so this Naked Pair attacks candidates in two units – very useful.  The pair is 1 and 4.  Looking around in box 7 we have both 1’s and 4’s and these can be scrubbed out.  Looking along the row we can do the same.  A lot of clutter disappears if you compare this with Figure 3.  Note a new Naked Pair (2/7) is revealed and a solution is available in the corner (J1).

Naked Pair Fiendish Exercise

There are 38 candidates that can be removed from this Sudoku board. You are allowed only to look for Naked Pairs and tick candidates off.  Certainly some removals will reveal new Naked Pairs.  Use them.