Advanced techniques
The cross-grid patterns behind the hardest puzzles.
Advanced — what "Nightmare" means
These patterns span multiple rows and columns at once. They're powerful but rare — most puzzles never require them. The hardest difficulty you can choose on Stillgrid is Nightmare, and a puzzle has to need techniques like these to earn it. (Play "Any" difficulty and you may also spot the grader's own "Hard" and "Diabolical" labels on individual puzzles.)
X-Wing
When a digit is a candidate in exactly two cells of one row, and in exactly two cells of a second row that line up in the same two columns, those four cells form a rectangle. The digit must occupy opposite corners — so it can be erased from those two columns everywhere else.
A candidate 5 appears in just two columns across two rows, forming a rectangle. 5 is then removed from those two columns in all other rows.
Swordfish, XY-Wing, and chains
Swordfish extends the X-Wing idea to three rows and columns. XY-Wing links three bivalue cells so a digit is eliminated where two of them see the same cell. Chains (coloring and alternating inference chains) follow strong/weak links between candidates to force eliminations. Stillgrid's grader uses all of these to label the very hardest puzzles — but generated puzzles rarely need them.