Sudoku

The grid is nine by nine, divided into nine three-by-three boxes. Fill it so that every row, every column, and every box contains the digits one through nine exactly once. That is the entire rulebook. The printed digits — the givens — are fixed; your job is the rest. There is no arithmetic in Sudoku, no adding or dividing. The digits could as well be nine kinds of seashell. It is a logic puzzle wearing a numerical costume.

Every puzzle we serve is generated fresh and then verified, mechanically, to have exactly one solution. This matters more than it sounds: it is the guarantee that pure reasoning will always carry you to the end. If you find yourself wanting to guess, there is always a better move available somewhere on the board — a hint can point to it, and undo is bottomless. The Tricky setting starts you with fewer givens and demands longer chains of reasoning, but the promise holds there too.

A short history

The family tree starts with Leonhard Euler, who studied what he called Latin squares in the 1780s — grids where each symbol appears once per row and column. The modern puzzle, with its three-by-three boxes, was the quiet invention of Howard Garns, a retired architect from Indiana, whose “Number Place” first ran in Dell puzzle magazines in 1979. Japanese publisher Nikoli picked it up in 1984 and gave it the name that stuck — sudoku, roughly “the digits must remain single.” When a New Zealand judge named Wayne Gould talked The Times of London into printing one in 2004, the puzzle went around the world in a season. Garns did not live to see it; his hometown paper ran the puzzle daily without knowing its inventor had lived up the road.

Advice that actually helps

• Scan, don't stare. Pick a digit that already appears often and sweep box by box: its row and column shadows frequently leave only one legal cell in a box. This single habit solves most of an easy puzzle.
• Hunt the crowded houses. A row, column, or box with six or seven digits placed will often hand you the rest for free. Finish what is nearly finished before wandering.
• Use pencil marks like a ledger, not graffiti. Mark candidates only in the region you are actively working. When a cell's marks shrink to one, that's a placement; when two cells in a unit share the same two marks, those digits are spoken for and can be crossed off elsewhere in that unit.
• Every placement is a small tax on its neighbours. After writing a digit, spend five seconds updating what it rules out. The players who seem fast are simply the ones who never pay this tax twice.

Common mistakes

The cardinal error is writing a digit because it could go somewhere rather than because it must. One optimistic placement poisons every deduction built on it, and the rot surfaces twenty moves later — which is why our Check button exists, and why it reports disagreements without fixing them. The second mistake is tunnel vision: chasing one stubborn digit around the board while three easy placements sit ignored in the next box. When you stall, change lenses — switch from following a digit to auditing a region. And the third is treating pencil marks as a chore for hard puzzles only. They are cheapest exactly when you are not yet desperate.