Twenty years ago in Lucca, 85 people from 22 countries sat down to settle who was best at sudoku. I spent this week reading about that weekend, partly because the nineteenth one is five weeks away. What stayed with me was how far back the winner stood after the first round.
Lucca, 2006
The first World Sudoku Championship was held in Lucca, Italy, in March 2006. Jana Tylová of the Czech Republic won it. She was thirty-one. She came ninth in the opening round and passed everyone ahead of her, and she was the only woman in the top eighteen. Thomas Snyder of the United States finished second that year and won the next two championships himself.
Asked afterwards how she had prepared, she said she had been solving about two hours a day, and her advice to everyone else was to practise every day and follow the sites where the puzzles are. That is the whole method. Nobody has found a better one in twenty years.
Kota Morinishi of Japan has four world titles, more than anyone, and the three most recent went to China: Tantan Dai in 2023 and again in 2025, Letian Ming in 2024. The nineteenth championship runs in Kolkata from October 11 to 15, hosted by Logic Masters India alongside the thirty-third World Puzzle Championship. So far 121 solvers from 24 countries have entered.
The hidden pair
Last week I taught the naked pair, which starts with two squares. The hidden pair starts with two digits.
Here is one from an orange belt board in the school. Row 3 has three givens: a 3 in column 2, a 4 in column 3, a 6 in column 8. Ask where an 8 can go along that row. The 8 at row 2, column 1 shuts the left box and the 8 at row 1, column 5 shuts the middle one, so the 8 sits in column 7 or column 9. The 9 is shut out of the same squares by the 9 at row 2, column 6 and the 9 at row 9, column 1. Column 7 or column 9 again.
Two digits, two squares. They fill those squares in one order or the other, so nothing else can. Row 3, column 7 was showing 1, 5, 7, 8 and 9, and it loses the 1, the 5 and the 7. Row 3, column 9 was showing 2, 7, 8 and 9, and it loses the 2 and the 7.
That erasure is the point. The 1 in row 3 had two homes, column 1 and column 7, and column 7 has just lost it. The 1 is certain in row 3, column 1, and it was certain before you could see it.
The usual mistake is to take the two digits out of the rest of the group and stop. That is the naked pair's move. A hidden pair works inside its own two squares.
This week's editions
Seven printed: No. 56 and No. 57 easy, No. 58 and No. 59 medium, No. 60 hard, No. 61 the Saturday master, and No. 62 expert on Sunday. The old newspaper ramp, gentle on Monday and brutal by Saturday.
Every dated edition since August 11 has been finished, 26 days running, 25 of them by at least two of you. Fourteen solvers have put 305 puzzles away between them, about 53 hours at the grid. The Saturday master has gone down ten times to five of you, four of those with no wrong entry and no hint at all.
In Claire's Sudoku
Monday and Tuesday are where a technique becomes yours. The board is gentle enough that you can stop in the middle, try what you just learned, and lose nothing if it takes a while. By Saturday the master asks for it under pressure, and that is when you learn whether you have it.
The hint is there for the one square on a Saturday night that will not come. Seven of the 80 finishes since August 11 used one, three of them on a master. Take it when you want it. The paper prints a new grid every morning either way.
One thing to carry out of this letter. On the next grid that stops you, pick two digits and mark every square in a group where each can still go. When both lists come back as the same two squares, you have a hidden pair, and everything else in those squares comes off.
The championship starts in Kolkata on October 11. I will tell you how it goes.