Unavoidable Set

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An unavoidable set is a Deadly Pattern from the point of view of a puzzle maker, or somebody who likes to investigate solution grids.

Take the following example:

. 1 .
. . 2
. . .
. 2 .
. 1 .
. . .
. . .
. . .
. . .
. 2 1
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .

If there were no givens in any of these 6 cells, the solver would eventually reach a point were a choice must be made how to place digits 1 and 2 in them. The conclusion would be that the puzzle has 2 possible solutions. It is unavoidable for the puzzle maker to place at least one given in one of these cells.