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Hidden Twins

The Hidden Twins method of is one of the easiest higher methods. While the Naked Twin is exposed, this one "hides" among other candidates. Using higher methods is worthwhile only when the two basic methods provide no further progress.

A Hidden Twin occurs when you find a group of two candidates which can only appear in the same two fields within a logical unit (row/column/block). Each of the two fields must contain both candidates and may also have additional candidates. The two candidates in the group may not appear in any other field in the logical unit.
This may sound complicated at first , but is often easier to see in practice than you might expect.

Since the two fields of the group must ultimately be filled with the two candidates of the group, all candidates that do not belong to group can be deleted from the fields of the group. In other words: the Twin's two fields are reduced to the Twin's two candidates.

In the pink highlighted fields, the candidates 1 and 3 make up a Hidden Twin.
This is reflected equally in the column as in the block.

In the green fields, the candidates 2 and 5 make up a Hidden Twin.
It is evident in both the column and the block.

Sudoku example hidden Twin

As a consequence, several candidates can be deleted from the yellow fields.

The Hidden Twin is a special case of the Hidden group.

Sudoku example hidden Twin

see also: Generalization : Hidden groups



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