Four cells whose combined candidates span exactly four digits. The restricted common candidate is eliminated from cells seeing all its possible positions.
A WXYZ-Wing is the largest member of the Wing family (XY-Wing → XYZ-Wing → WXYZ-Wing). It involves four cells whose combined candidates cover exactly four digits {W, X, Y, Z}. One digit among these four is the "restricted common", it appears as a candidate in cells that are all visible to the elimination targets.
The logic extends naturally from smaller wings. If four cells contain only four digits between them, each digit must go in exactly one of those cells. Any cell that sees every possible position of a particular digit among the four cells cannot contain that digit, it's guaranteed to be in one of the cells it sees.
WXYZ-Wings are rare and hard to spot in practice. They represent an advanced pattern that most solvers encounter only in the hardest puzzles. Don't try to hunt for these, but recognize the logic if your solver or hint system points one out.
Find four cells whose combined candidate set contains exactly four digits. The cells don't all need to share a single unit, but the pattern must be connected, typically a pivot cell seeing three wings, or cells linked through shared units.
Identify the restricted common candidate: the digit that appears in a subset of the four cells where all those cells share a common unit (so they can be "seen" together). This is the digit you will eliminate.
Any cell outside the pattern that can see every cell in the pattern containing the restricted digit cannot hold that digit. It's guaranteed to be in one of those pattern cells.
Example 1: WXYZ-Wing
Four cells form the pattern: R7C6 {2, 4, 5, 6}, R5C6 {2, 4, 5}, R6C6 {2, 4}, and R8C5 {4, 5}. Their combined candidates span {2, 4, 5, 6}, exactly four digits in four cells.
The restricted common candidate is 4, it appears in cells that are all visible to the elimination targets. R8C6 and R9C6 see all positions where 4 can appear among the pattern cells.
Eliminate digit 4 from R8C6 and R9C6.
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