Sudoku Techniques Part Two: X-Wings, Swordfish, and Other Advanced Patterns
If you’ve mastered the basics of Sudoku — scanning, pencil marks, naked pairs, hidden singles — you’ve probably hit a new wall. The puzzles labeled ‘hard’ or ‘expert’ on browser sites like Situs YYPAUS sometimes can’t be solved with basic techniques alone. They require advanced patterns. Most casual players never learn these patterns and either give up on hard puzzles or solve them by trial-and-error. The patterns themselves are surprisingly learnable.
X-Wing
X-Wing is the first advanced pattern most players learn, and it’s deeply useful. Look for a digit that appears as a candidate in exactly two cells of two different rows — and those cells fall in the same two columns. The pattern forms a rectangle. The digit must occupy two of the four corners (one per row and one per column), which means you can eliminate that digit from any other cells in those two columns.
Why X-Wing works
The logic: each row needs the digit in one of its two candidate cells. Because of the row constraints, the digit ends up in one corner of one row and the opposite corner of the other row — but always in those two columns. Any cell elsewhere in those columns is therefore safe to eliminate.
Swordfish
Swordfish is X-Wing’s three-row cousin. Look for a digit appearing as a candidate in two or three cells of three rows, where all those cells fall within the same three columns. The digit must occupy one cell per row and one cell per column. You can eliminate that digit from any other cells in those three columns.
Jellyfish
Jellyfish extends the pattern to four rows and four columns. It’s rare and rarely necessary — most puzzles that contain Jellyfish situations also contain easier solutions elsewhere. Worth knowing it exists but not worth grinding for.
XY-Wing
Look for three cells with exactly two candidates each, where the three cells share specific candidate overlaps. If cell A has candidates 1 and 2, cell B has candidates 2 and 3, and cell C has candidates 1 and 3, and A ‘sees’ both B and C (shares a row, column, or box with each), then any cell that sees both B and C cannot contain 3.
Unique Rectangle
If you find four cells forming a rectangle, all sharing the same two candidates, in two rows and two columns that span only two boxes, then a ‘deadly pattern’ exists. Because well-constructed Sudoku puzzles have unique solutions, this pattern can’t actually occur — which means one of those candidates must be eliminated somewhere to prevent it.
Practice with marked puzzles
These techniques look complex when described but become intuitive with practice. Some browser Sudoku versions include hint systems that point out X-Wings when they exist. Use the hints to learn what the patterns look like, then turn the hints off and find them yourself.