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 from smaller wings, but the four cells are NOT a locked set (they don't all see each other), so it is not true that each digit goes in exactly one cell. Instead, the pattern works when every one of the four digits EXCEPT one is "restricted" — confined so that all of its positions among the four cells mutually see each other. The single leftover digit, the non-restricted common candidate Z, is then forced into one of the four cells, so any cell seeing all of Z's positions cannot itself be Z.
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 candidates: the shared digits whose occurrences among the four cells all see each other (share a common unit), so at most one cell can actually hold each. These are the forcing digits and are NOT eliminated. The one remaining shared digit whose positions do NOT all see each other is the non-restricted common Z — that is the digit you 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: pivot R6C3 {2, 3, 6, 8}, and wings R4C3 {3, 6}, R5C1 {2, 6}, R7C3 {6, 8}. Their combined candidates are exactly {2, 3, 6, 8}, four digits across four cells.
Three of the digits are restricted — each is confined so its positions mutually see: 2 (in R6C3 and R5C1, sharing box 4), 3 (in R6C3 and R4C3, sharing column 3), and 8 (in R6C3 and R7C3, sharing column 3). That leaves 6 as the non-restricted common, which must land in one of the four cells.
R5C3 sees all four pattern cells — R4C3, R6C3 and R7C3 down column 3, and R5C1 along row 5. Since one of those four must be 6, eliminate 6 from R5C3.
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