Strong links and three-cell wings

Alternating inference chains (AICs) built from strong-link nodes and weak inferences, plus small ALS constructions such as XY-Wing and XYZ-Wing.

Skyscraper

Two parallel strong links with endpoints that force an elimination.

2-String Kite

A row and column strong link connected through a box.

Two strong links connect four candidate 2s through a weak link in a Sudoku grid

2 Strong Links

Two conjugate pairs joined by a weak link.

Turbot Fish

A compact single-value forcing chain with the Turbot Fish structure.

XY-Wing

A three-bivalue-cell AIC and ALS construction with a shared endpoint digit.

XYZ-Wing

A three-cell ALS-XZ construction whose shared digit has three possible sources.

Strong links, conjugate pairs and wing logic

For one candidate digit, a strong link exists when a house contains exactly two possible positions: if one is false, the other must be true. Skyscrapers, 2-String Kites, 2 Strong Links and Turbot Fish connect conjugate pairs through cells that see one another. Their shapes differ, but their conclusion is the same: at least one endpoint must contain the digit, so any outside cell that sees both endpoints cannot contain it.

XY-Wing is a three-bivalue-cell AIC and ALS construction. XYZ-Wing is a three-cell ALS-XZ construction with one trivalue cell. Their shared digit can be removed only from a candidate that sees every possible occurrence guaranteed by the proof. The candidate sets and visibility define the deduction; the drawing merely illustrates it.

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