Sudoku Techniques

Free guides to every Sudoku solving technique, from full house and naked singles to X-Wing. Each explains how to spot the pattern and use it, with an interactive lesson to practice.

• Full House — The last empty cell in a unit can only be one digit.
• Naked Single — A cell has only one candidate left after eliminating peers.
• Hidden Single — A digit fits only one cell within a unit.
• Naked Pair — Two cells in a unit share the same two candidates.
• Naked Triple — Three cells in a unit share three candidates between them.
• Hidden Pair — Two digits fit only the same two cells in a unit.
• Hidden Triple — Three digits fit only the same three cells in a unit.
• Naked Quad — Four cells in a unit share only four candidates.
• Hidden Quad — Four digits fit only the same four cells in a unit.
• Pointing Pair — A digit confined to one box line eliminates it along that line.
• Box-Line Reduction — A digit confined to one line within a box clears the box.
• XY-Wing — A pivot and two wings force a digit out of the cells they all see.
• XYZ-Wing — A three-candidate pivot and two wings share a removable digit.
• W-Wing — Two matching pairs linked by a strong link remove a digit.
• Skyscraper — A digit in two lines sharing one base clears the roofs’ peers.
• Two-String Kite — A row link and a column link joined in one box remove a digit.
• Empty Rectangle — A digit boxed into one row and column, plus a conjugate pair.
• Unique Rectangle — Three corners sharing a pair would force two solutions.
• BUG+1 — An almost-all-bivalue grid forces its one tri-value cell.
• X-Wing — A digit forms a rectangle across two rows and two columns.
• Swordfish — A digit confined to three columns across three rows clears them.
• Jellyfish — A digit confined to four columns across four rows clears them.
• Remote Pairs — A chain of identical two-candidate cells colours into two answers.
• Simple Colouring — Two-colour a single digit’s conjugate chain to force eliminations.
• X-Chain — A single-digit chain of alternating links forces one end to be true.
• XY-Chain — A chain of pairs that starts and ends on one digit forces an end.
• Finned X-Wing — An X-Wing with one extra in-box candidate that narrows the cut.
• Finned Swordfish — A Swordfish with an in-box fin — and its Sashimi degenerate form.
• ALS-XZ — Two almost-locked sets linked by a restricted candidate lock a digit.
• Sue de Coq — A box/line intersection splits its digits between the line and box.
• ALS-XY-Wing — Three almost-locked sets chain through a pivot to trap a digit.
• AIC — An alternating inference chain proves one of its two ends is true.
• Grouped AIC — An alternating inference chain that links through box/line groups.
• Death Blossom — A stem cell whose candidates each link to an almost-locked set.
• Forcing Chain — A candidate that forces a contradiction can be ruled out (Nishio).