Expert Sudoku – X-Wing, XY-Wing and Swordfish puzzles
Mistakes
0
Time
0:00
Preparing a expert puzzle…
Embed this game on your site
Free for blogs, schools, libraries and clubs. Paste this HTML where you want the game to appear; please keep the credit link.
Quick answer
Sudoku is a free number-placement logic puzzle that plays instantly in your browser on phones, tablets and computers, with no download, sign-up or ads. Fill the 9×9 grid so every row, column and 3×3 box holds the digits 1 to 9 once.
Sudoku at a glance
What it is
Number-placement logic puzzle
This page
Expert level: X-Wing, XY-Wing or Swordfish
Goal
Fill the 9×9 grid so every row, column and 3×3 box holds the digits 1 to 9 once
Modes
Easy, Medium, Hard and Expert, graded by the hardest solving technique needed; every puzzle has exactly one solution
Controls
Tap a cell and a number, or type 1-9; notes mode, auto notes, mistake highlighting, hints, undo
Players
1
Price
Free, no ads, no sign-up, no download
Works on
Phones, tablets and computers, in any modern browser
Expert puzzles are built to need at least one advanced pattern: an X-Wing, an XY-Wing or a Swordfish. They are still fully logical. Every puzzle is solved step by step by the same engine that powers the hints before you see it, so there is always a way forward without guessing.
The three expert techniques
X-Wing. Pick a digit and look at the rows. If the digit has exactly two possible cells in row A and exactly two in row B, and they line up in the same two columns, the digit fills two diagonal corners of that rectangle. Remove it from every other cell in those two columns. The same applies with rows and columns swapped.
Swordfish. An X-Wing across three rows and three columns. Each row has the digit in two or three places, all inside the same three columns. Remove the digit from the rest of those columns.
XY-Wing. Find a pivot cell with exactly two candidates, X and Y. Find two cells it can see: one with X and Z, one with Y and Z. Whatever the pivot turns out to be, one of those two cells must be Z, so any cell that sees both of them cannot be Z.
Strategy tips
Keep your notes accurate. A single missing candidate can hide or fake a pattern. Fill notes resets everything from the current grid.
Hunt one digit at a time for fish. Highlight a digit by tapping it in the grid and look for rows where it appears exactly twice.
Look for cells with two candidates. They are the raw material of XY-Wings.
Use the hint to learn. It shows the next cell you can fill and names the technique that unlocks it, so you can go back and find the pattern yourself.
How to play Sudoku
1
Fill in candidates
Press Fill notes. At expert level you need a full, accurate set of pencil marks to see the patterns.
2
Exhaust the simpler techniques
Work through singles, locked candidates and pairs until nothing more falls. Expert steps are only needed when those stop.
3
Look for an X-Wing
Find a digit that has exactly two places in each of two rows, in the same two columns. Remove it from the rest of those columns.
4
Look for an XY-Wing
Find a cell with two candidates XY that sees a cell XZ and a cell YZ. Any cell that sees both of those cannot be Z.
Fewer clues and real candidate work. You will need locked candidates, pairs or triples.
Frequently asked questions
What makes an expert puzzle?
+
Every expert puzzle needs at least one X-Wing, XY-Wing or Swordfish to finish. Puzzles that can be solved with easier techniques are rejected, so expert never turns out to be just a hard puzzle.
Are expert puzzles fair?
+
Yes. Each one is solved by a logic engine using only these named techniques before it reaches you, so there is always a logical path and exactly one solution. No guessing or trial and error is needed.
What is an X-Wing?
+
Take one digit. If in two rows it can only go in the same two columns, it must sit in two opposite corners of that rectangle. Either way, both columns are covered, so the digit can be removed from every other cell in those two columns. It works with rows and columns swapped too.
What is a Swordfish?
+
The same idea as an X-Wing with three rows and three columns. If a digit's places in three rows all fall within the same three columns, remove it from the rest of those columns.
Why does the puzzle take a moment to appear?
+
Expert puzzles are rare among random grids, so the generator may try several before it finds one that needs an expert technique. It runs in the background, and the next puzzle is prepared while you play.
Fancy a different puzzle?
Minesweeper is logic of another kind: read the numbers and clear the field without opening a mine.