Sudoku is solved through logic, not guessing. Every puzzle on this platform, from Easy to Diabolical, can be cracked using the techniques below. Easy puzzles need only Naked Singles. Medium adds Hidden Singles and Locked Candidates. Hard introduces Pairs, Triples, and Unique Rectangles. Extreme brings X-Wings, Skyscrapers, and Coloring. Diabolical demands XY-Chains, Finned Fish, and ALS.
Select any technique below to learn it with step-by-step guides and worked examples. Practice in Training Mode, where the Next Step button guides you through each deduction.
Easy Easy Puzzles
Naked Single — A cell where all candidates except one have been eliminated by its row, column, and box
Medium Medium Puzzles
Hidden Single — A digit that has only one possible cell within a row, column, or box
Locked Candidates — When a digit in a box is restricted to one row or column, it can be eliminated from that row or column outside the box
Hard Hard Puzzles
Naked Pair — Two cells in a unit containing the same two candidates, so those digits are eliminated from other cells in the unit
Hidden Pair — Two digits that only appear in two cells within a unit, so all other candidates are removed from those cells
Naked Triple — Three cells in a unit whose combined candidates contain at most three digits, eliminating those digits elsewhere in the unit
Hidden Triple — Three digits that only appear in three cells within a unit, so all other candidates are removed from those cells
Naked Quad — Four cells in a unit whose combined candidates contain at most four digits. Rare, but follows the same logic as pairs and triples
Hidden Quad — Four digits confined to four cells in a unit. Extremely rare in standard puzzles
BUG+1 — When every unsolved cell has exactly two candidates except one with three, that cell's extra candidate is the answer
Unique Rectangle — A pattern of four cells across two boxes where eliminating candidates prevents the puzzle from having two solutions
Extreme Extreme Puzzles
X-Wing — A digit confined to two cells in each of two rows, sharing the same columns. Eliminates that digit from other cells in those columns
Skyscraper — Two conjugate pairs of a digit sharing one endpoint. The other two endpoints create an elimination
Two-String Kite — A row conjugate pair and a column conjugate pair connected through a shared box. Endpoints eliminate a candidate
Empty Rectangle — A digit's candidates in a box form a line pattern that combines with an outside conjugate pair for an elimination
Simple Coloring — Assign two colors to a chain of conjugate pairs. If two same-colored cells conflict, that color is false
XY-Wing — A pivot cell with two candidates connects to two wing cells. The shared candidate is eliminated from cells seeing both wings
Swordfish — A digit confined to 2-3 cells in each of three rows, all within three columns. Eliminates from other cells in those columns
W-Wing — Two cells with identical candidates connected by a strong link on one candidate. The other candidate is eliminated from cells seeing both
Diabolical Diabolical Puzzles
X-Cycles — Alternating strong and weak links for a single digit forming chains that reveal contradictions or placements
XY-Chain — A chain of bivalue cells where each shares a candidate with the next. The chain endpoints eliminate their common candidate
XYZ-Wing — Like XY-Wing but the pivot has three candidates. Eliminations apply to cells that see all three pattern cells
WXYZ-Wing — Four cells whose combined candidates total four digits. The common candidate is eliminated from cells seeing all four
Finned X-Wing — An X-Wing with one extra candidate cell. Eliminations are restricted to cells that also see the fin
Finned Swordfish — A Swordfish with an extra fin cell. Eliminations restricted to cells seeing both the Swordfish columns and the fin's box
Jellyfish — A digit confined to 2-4 cells in each of four rows, all within four columns. The largest standard fish pattern
ALS-XZ — Two Almost Locked Sets connected by a restricted common candidate. The other common candidate is eliminated from cells seeing both sets
Remote Pairs — A chain of cells sharing the same two candidates where alternating parity forces eliminations at both ends
Killer Killer Sudoku Techniques
Killer Sudoku keeps every Classic rule and adds cages — dashed groups of cells with a target sum and no repeated digit. These lessons walk each cage technique through the live Next-Step teacher, the same engine the in-game hint uses.
Cage Combinations — List the digit sets that can fill a cage, then drop the digits no combination uses.
Cage Must-Contain — When every valid combination of a cage shares a digit, that digit is trapped inside the cage.
Cage Naked Subset — A locked pair, triple, or quad inside a cage claims its digits, freeing the cage’s other cells.
Rule of 45 (Innies & Outies) — Every row, column, and box sums to 45 — the leftover “innie” or “outie” cells have a known total.