Sudoku
Classic 9×9, with hints that name the technique instead of giving the digit.
How to play Sudoku
Fill the grid so every row, every column and every 3×3 box holds the digits 1 to 9 once each. The digits that start on the board are fixed.
The techniques behind the hints
Each hint is one step a person could take, named after the pattern it uses.
- Naked single: a square has only one digit left after you cross off everything in its row, column and box.
- Hidden single: in a row, column or box, a digit has only one square where it can go.
- Pointing and claiming: a digit is stuck in one line inside a box, or in one box inside a line, which clears that digit from the rest of the line or box.
- Pairs and triples: two or three squares in a unit share only that many digits, so those digits leave the other squares.
- X-Wing: a digit has just two places in each of two rows, and the places line up in the same two columns. The digit is then gone from everything else in those columns. There is a worked example with pictures.
Questions
What do the difficulty levels mean?
Easy boards need only singles and start with 40 clues. Medium boards start with 32 and add locked candidates, and about a quarter also need a pair. Hard boards start with 26 to 28 clues and nearly all need pairs or triples. About 1 in 30 needs an X-Wing.
Will I ever have to guess?
No. A solver checks that exactly one solution exists, and a second solver that works like a person has to finish the board with the techniques the hints name. If it cannot, the board is thrown away.
What are the notes for?
Notes are small digits that record what could still go in a square. Switch on the pencil, tap a digit to add or remove a note, or press auto-notes to fill them all in and then cross out as you learn more.
Is my progress saved?
Your board, notes, timer and streak stay in your own browser.