Crane

Crane is the second in a trio of Sudoku solving techniques that consist of two strong links, and one weak link.

If you're not familiar with Strong and Weak links, you may find it useful to read about those first.

In this example, the '8' candidate appears twice in column 1, this means that if one of these '8' candidate is false, the other '8' candidate must be true, i.e. this is a strong link.

The '8' candidate appears twice in the central region, another strong link.

The '8' candidate appears more than twice in row 6, this means that if one of these '8' candidate is true, the other '8' candidates must be false, i.e. this is a weak link.

If we assume the '8' in cell A is false,

⇒ The 8 in cell B is true.

⇒ The 8 in cell C is false.

⇒ The 8 in cell D is true.

Separately, if we assume the '8' in cell D is false,

⇒ The 8 in cell C is true.

⇒ The 8 in cell B is false.

⇒ The 8 in cell A is true.

This means that the value in cell A or cell D (or both) must be '8'.

If the value of Cell A is '8', we would remove all '8' candidates from these cells.

If the value of Cell D is '8', we would remove all '8' candidates from these cells.

If the value of cell A or D (or both) must be '8', this means we can the '8' candidates where these two areas overlap.

There will always be two cells where these areas overlap, but only one of them could contain a candidate we can remove - if the cell in the first column had the value '8' as a candidate, the link between cells A and B would be a Weak link instead, and there wouldn't be a Crane here at all.