Master
XY-Chain
In a chain of two-candidate cells, one end holds its digit, so a cell that sees both ends cannot hold it.
Start at a cell with two candidates: if it isn't A, it's B. A two-candidate cell it sees that also holds B is then its other digit, and so on, cell by cell. If the last cell comes out as A, one end or the other is A, so remove A from the cells that see both ends.
This example, step by step
- Where to lookA pointer first: link the marked cells into a chain through 1, 5, 6, and 9.
- XY-ChainFollow the chain: either row 5, column 7 is 9 or row 9, column 6 is 9, at least one of them. So no cell that shares a group with both ends can be 9.
In hints
The hint first gives a direction. Blue marks the cells of the chain one by one, and orange strikes remove the digit from the cells that see both ends.