Techniques › XY-Chain › Practice
Practise the XY-Chain
A run of two-candidate cells that returns to the same digit at both ends.
8 real positions, each taken from an actual puzzle at the moment this was the cheapest move available. Two steps: find the cells that form the pattern, then find the candidate it rules out. A near miss tells you how many you had right, so getting it wrong is worth something.
| 7 | 2 | 9 | 5 | 3 | 1 | 6 | 4 | 8 |
| 3 | 6 | 4 | 2 | 8 | 7 | 9 | 1 | 5 |
| 5 | 1 | 8 | 9 | 4 | 6 | 3 | 2 | 7 |
| 89 | 3 | 5 | 1 | 79 | 28 | 27 | 6 | 4 |
| 189 | 7 | 6 | 4 | 59 | 28 | 12 | 58 | 3 |
| 18 | 4 | 2 | 6 | 57 | 3 | 17 | 58 | 9 |
| 4 | 8 | 1 | 7 | 6 | 9 | 5 | 3 | 2 |
| 6 | 5 | 7 | 3 | 2 | 4 | 8 | 9 | 1 |
| 2 | 9 | 3 | 8 | 1 | 5 | 4 | 7 | 6 |
How to spot it
List the bivalue cells. Start at one, step to a bivalue neighbour sharing a digit, carry the other digit forward, and keep going. When the carried digit matches the one you began with, check what both ends can see.
Full explanation on the XY-Chain reference page.
Practise something else
- XY-Wing A pivot and two pincers. Whichever way the pivot falls, one pincer takes the digit.
- Simple Colouring Follow one digit's forced chain through the grid in two alternating colours.