Advanced Sudoku patterns

A conjugate pair is a digit that appears as a candidate in exactly two cells of one house. That binary link (if not A then B) is the strong edge behind most AIC-style chains.

Two rows (or columns) each hold digit d in exactly two cells; one base column aligns while the peaks sit in different columns. Any cell that sees both peaks can drop d. Same geometry family as a short X-chain.

Two bivalue cells share the same pair {x,y}. A strong link on x (or y) connects them through other cells. Then y (or x) can be eliminated from cells that see both bivalue endpoints.

Y-Wing: a bivalue pivot and two bivalue arms that share one digit with the pivot; the common wing digit falls from cells seeing both arms. XYZ-Wing: the pivot holds three candidates {x,y,z}; z drops from cells that see the whole triple of nodes.

Inside one box, candidates for d form a “broken” rectangle (present in two lines of the box, missing a corner). Paired with a conjugate of d outside the box, you can eliminate d from a cell that sees both the external conjugate and the open side of the box pattern.

An almost-locked set in a box overlaps an almost-locked set in a line. Digits locked by that overlap can be cleared from the rest of the box and the rest of the line. Rare on easy grids; common vocabulary in hard-grid writeups.

A chain of cells that all contain the same bivalue pair {a,b}, linked by conjugates. Cells that see both an “even” and an “odd” link in the chain can drop both a and b.

Color conjugate networks for one or more digits. A color that produces a contradiction (same color twice in one house, or both colors in one cell) is false; the opposite color set becomes true. Useful when simple AIC gets crowded.

Ordinary X-chains step cell to cell on one digit. Grouped links treat several candidates in one box as a single node, so the chain can turn through a box group that a single-cell chain could not use.

Four cells in one house whose candidates lie inside the same four-digit set form a naked quad — those four digits leave the rest of the house. Humans miss them because four cells must be scanned at once; solvers catch them with bitset popcounts.

Type 1 clears the extra digit on a potential deadly rectangle. Higher types treat extra candidates on the rectangle as a virtual locked set with other cells in the house, then eliminate those extras elsewhere. Always assume the published grid is meant to be unique.

A base pair of cells and distant target cells lock a small digit set into a corridor across bands or stacks. When the pattern fits, those digits can be stripped from adjacent pathways. Spotting it by hand is uncommon; named solvers document examples.

If you have already cleared singles, pairs, fish, and a short chain, a step explainer (SudokuWiki and similar tools under Solvers & tools) is faster than hunting Exocet by eye. Ask for the next elimination reason — not a full paste.