The Sudoku blog
Short reads about puzzles, solving, and how this site works.
- Why every puzzle here has exactly one answer
How the puzzles on this site are made, why a proper Sudoku can only be finished one way, and what the difficulty levels really change.
- When to start using pencil marks
Notes are the most useful tool in Sudoku and also the easiest to overdo. Here is when to use them, how much to write, and how to keep them honest.
- Seventeen: the fewest clues a Sudoku can have
A Sudoku needs at least 17 given numbers to have only one answer. Here is what that means, how it was proved, and why our Extreme puzzles stop at around 22.