WXYZ-Wing: find the one non-restricted digit
In this Type 1 WXYZ-Wing, four cells use exactly four digits. For three digits, every occurrence within the pattern sees every other occurrence of that digit. The fourth digit is non-restricted: at least two of its occurrences do not see each other.
Key idea
If the non-restricted digit were absent, the other three digits could each fill at most one pattern cell, leaving four cells with only three available placements. So at least one pattern cell contains the non-restricted digit. Remove it only from an outside cell that sees every pattern occurrence of it.
See the move, step by step
Use R1C1 {1,2}, R1C4 {2,3}, R1C7 {3,4}, and R2C1 {1,4}. This constructed candidate state includes earlier deductions.
Step 1 of 4
Find four cells using four digits
The four pattern cells collectively use only 1, 2, 3, and 4. R1C1 and R2C1 share box 1; the other connections lie along row 1.
Step 2 of 4
Identify the non-restricted candidate
The 1s share box 1, the 2s share row 1, and the 3s share row 1. The 4s at R1C7 and R2C1 do not see each other, so 4 is the only non-restricted digit.
Step 3 of 4
Find a peer of both possible 4s
R2C7 sees R1C7 through column 7 and R2C1 through row 2. At least one of those pattern cells must be 4, so remove 4 from R2C7.
Step 4 of 4
Keep the four-cell set
R2C7 now contains only 5 and 6. The four pattern cells retain their candidates; the wing does not determine which one contains 4.
Try it on today's puzzle
Check every digit's peer relationships. This example requires exactly one non-restricted digit; four cells with four digits alone are not enough. The target must see every pattern occurrence of the digit removed.
Play today