Master Sudoku

Sudoku is played on a grid of 9 rows and 9 columns, split into nine 3×3 boxes. Some cells start with a digit (the clues). You fill in the rest.

Master puzzles start with as few clues as our generator could remove while keeping a single solution.

• Each row holds the digits 1 to 9, each exactly once.
• Each column holds the digits 1 to 9, each exactly once.
• Each 3×3 box holds the digits 1 to 9, each exactly once.
• The clues can't be changed. The puzzle is solved when every cell is filled and no digit repeats in a row, column or box.

Each master puzzle gets stuck after everything the easier levels use: singles, pointing, pairs, triples, quads and the X-Wing. It needs at least one of an XY-Wing, an XYZ-Wing, a Swordfish or a Jellyfish.

Of our 1000 master puzzles, our solver needed an XY-Wing in 918, an XYZ-Wing in 128, a Swordfish in 26 and a Jellyfish in 2. Most also need the expert techniques on the way. No puzzle needs trial and error: our solver finished every one with named techniques.

XY-Wing: find a cell with two candidates, X and Y (the pivot). It sees one cell with X and Z and another with Y and Z. If the pivot is X, the first cell is Z; if it is Y, the second cell is Z. So any cell that sees both of those cells can't be Z.

XYZ-Wing: the pivot has X, Y and Z, and sees cells with X and Z and with Y and Z. Z can be removed from any cell that sees all three.

Swordfish: one digit, three rows, and on those rows the digit fits only in the same three columns (two or three places per row). The columns are then covered, and the digit leaves the rest of them. Jellyfish is the same with four.

Pick a cell, then a number; Notes switches the buttons to pencil marks (N on a keyboard). Turn off Show mistakes in Settings if you want to check your own work, as master players often do. Undo takes back any step, and your game is saved in this browser.

Keep complete notes and clear them after every digit you place: wings live in cells with two notes, and a stale note hides them.

When the easy steps run out, list the cells with exactly two notes and test each as a pivot. That finds most XY-Wings. For fish, go digit by digit and write down which columns each row allows.

Hint names the technique the next step needs and marks its cells. Our technique guides show each pattern on a real puzzle from this site.