Learn
Learn each technique from the Sudoku rules up — why it must be true — with real mid-solve boards beside a short walkthrough.
Difficulty
Move type
Open Singles
A house with exactly one empty cell. Place the only digit that is still missing in that house.
Naked Single
A cell whose pencil marks shrink to a single digit. Place that digit in the cell.
Hidden Single
A digit that appears as a candidate in only one cell of a house. Place that digit in that cell.
Naked Pair
Two cells in one house that both contain only the same pair of digits. Eliminate those two digits from all other cells in the house.
Pointing Elimination
A digit in a box confined to one row or one column. Remove that digit from the rest of the line outside the box.
Hidden Pair
Two digits that both appear only in the same two cells of a house. Remove every other candidate from those two cells.
Hidden Triple / Quad
N digits that appear only in the same N cells of a house. Remove every other candidate from those N cells.
X-Wing
Two rows where a digit sits in the same two columns (or vice versa). Eliminate that digit from those columns (or rows) outside the two lines.
Skyscraper
Two almost-aligned strong links on one digit that share a base line. Eliminate that digit from cells that see both rooftop ends.
Two-String Kite
A row strong link and a column strong link on one digit that share a box. Eliminate that digit from cells that see both far ends.
Crane
Two strong links on one digit joined by a weak link. Eliminate that digit from cells that see both chain ends.
Swordfish
Three rows where a digit sits only in the same three columns (or vice versa). Eliminate that digit from those columns (or rows) outside the three lines.
Y-Wing
A bi-value pivot XY with two bi-value wings XZ and YZ. Eliminate Z from cells that see both wing cells.
XYZ-Wing
A three-candidate pivot XYZ with bi-value wings XZ and YZ. Eliminate Z from cells that see the pivot and both wings.
W-Wing
Two identical bi-value XY cells connected by a strong link on one digit. Eliminate the other digit from cells that see both XY cells.
Empty Rectangle
A box where a digit’s candidates leave an empty rectangle, plus a strong link outside. Eliminate that digit from the cell that sees both the link and the rectangle.
Unique Rectangle
A potential deadly rectangle on two digits with extra candidates on corners. Eliminate candidates that would force a non-unique solution.
Simple Colouring
A conjugate network on one digit that creates a colouring contradiction or trap. Eliminate the digit using same-colour or opposite-colour rules.
Finned X-Wing
An X-Wing shape with one extra fin candidate in a box. Eliminate that digit from cells covered by the fin.
WXYZ-Wing
A four-cell wing group on four digits with a common elimination digit. Eliminate that common digit from cells that see all Z locations.
Finned Swordfish
A near-swordfish on one digit with fin candidates in a box. Eliminate that digit from cells covered by the fin.
Jellyfish
Four rows where a digit sits only in the same four columns (or vice versa). Eliminate that digit from those columns (or rows) outside the four lines.
BUG+1
A nearly all bi-value grid with exactly one three-candidate cell. Place the extra candidate in that three-candidate cell.
Finned Jellyfish
A near-jellyfish on one digit with fin candidates in a box. Eliminate that digit from cells covered by the fin.
X-Chain
A nice loop or open chain of conjugates on one digit. Eliminate that digit from cells that see both chain ends.
XY-Chain
A path of bi-value cells linking the same digit at both ends. Eliminate that digit from cells that see both endpoints.
3D Medusa
A multi-digit colouring network with a contradiction or trap. Eliminate candidates using 3D colouring rules.
AIC
A strong–weak alternating chain that closes on a useful elimination. Eliminate candidates that see both strong endpoints.
Nishio
A candidate whose trial forces a contradiction. Eliminate the candidate that leads to impossibility.