<< The Swordfish technique <<
The XY-Wing technique
The XY-Wing or Y-Wing technique sounds similar but is different. It is based on a kind of forcing chain consisting of two links for candidates.
Let’s have a look on an example.
Here the marked cells are involved and assuming to enter a number will force the next to decide for a number too.
We start assuming that cell 1,2 will get value 6. That would force cell 1,8 to take number 9 (because it then cant take 6).
That again would force cell 2,9 to take value 5 (because it then cant take 9).
Finally that would mean for our target cell 2,1 that it can not take 5 (because it is on the same row).
Ok, when going back to start we could also assume that cell 1,2 takes 5 (instead of 6).
Ether way that would also force cell 2,1 to not take 5.
In conclusion we see it doesn’t matter whether cell 1,2 takes 5 or 6. Ether way cell 2,1 cant take 5 and we can:
- remove candidate 5 from cell 2,1
>> The XY-Chain technique >>