Advanced Sudoku Techniques

An X-wing is a pattern where a digit appears as a candidate in exactly two cells of two different rows, and those four cells line up across the same two columns.

A swordfish is the three-row generalisation of X-wing.

A jellyfish is the four-row generalisation of X-wing (and swordfish).

An XY-wing is a three-cell pattern.

An XYZ-wing extends XY-wing.

Coloring is a single-digit chain technique.

A forcing chain picks a cell with two candidates, assumes each candidate in turn, and follows the deductions.

A unique rectangle is a four-cell pattern at the corners of a rectangle, spanning exactly two boxes, where three corners have the same two candidates {X, Y}.

A BUG (bivalue universal grave) pattern arises when every empty cell in the grid would have exactly two candidates.

Nishio picks a single candidate, tentatively places it, and follows forced deductions until either a contradiction or a valid completion appears.