Hard Sudoku
Hard Sudoku puzzles need a pattern technique: X-Wing, Swordfish, XY-Wing, XYZ-Wing or a naked or hidden quad. Keeping candidate notes makes these patterns much easier to spot.
Hard puzzles need at least one of these techniques, and may use any easier one:
- X-Wing
- A digit is limited to the same two columns in two rows (or the reverse), so it leaves the rest of those lines.
- XY-Wing
- Whatever the middle cell holds, one of the two wings gets the shared digit, so cells seeing both wings can't.
- Naked quad
- Four cells hold only four digits between them, so those digits leave the rest of their area.
- Hidden quad
- Four digits fit only in the same four cells, so those cells hold nothing else.
- Swordfish
- A digit is limited to the same three columns in three rows (or the reverse), so it leaves the rest of those lines.
- XYZ-Wing
- One of these three cells must hold the shared digit, so cells seeing all three can't.
How to play
Sudoku is a number puzzle on a 9×9 grid divided into nine 3×3 boxes. Fill every empty cell with a digit from 1 to 9 so that each row, each column and each box contains every digit exactly once.
Every Sudoku here has exactly one solution, checked by a separate solver before it ships, and none of them needs a guess. The level is set by the hardest technique the puzzle truly needs, not by how many digits are given.
- Each row contains the digits 1 to 9 once.
- Each column contains the digits 1 to 9 once.
- Each 3×3 box contains the digits 1 to 9 once.
- The given digits cannot be changed.
Playing on this site
- Select a cell, then type a digit or tap it on the number pad. Arrow keys move the selection.
- Turn on Notes (or press N) to pencil in candidates instead of answers.
- Hint first points out any wrong digits, then names the next technique and shows where it applies before giving anything away.
- Your puzzle is saved in this browser, so you can come back to it later.