Sudoku

One puzzle a day

One puzzle a day, the same grid for every player. Your first run at it is the one that counts: solve it and your streak grows, your time goes on record, and a clean finish earns a perfect win. After that the play button keeps dealing fresh grids, as much practice as you want, none of it touching your records. Good for warming up a technique, or for chasing a faster time with nothing riding on it.

What is Sudoku?

A logic puzzle on a 9×9 grid divided into nine 3×3 boxes. The grid starts partly filled, and your job is to place the digits 1 through 9 so that every row, every column and every 3×3 box holds each digit exactly once. There's no arithmetic in it at all, despite the numbers. It's deduction from start to finish. A well-formed Sudoku has exactly one solution, and that solution can always be reached without guessing.

A brief history

The Japanese publisher Nikoli popularised modern Sudoku in 1986 under the name Sūji wa dokushin ni kagiru, roughly "the digits must remain single." Its mathematical roots go back further, to the Latin squares Leonhard Euler studied in the eighteenth century, and a direct precursor called Number Place ran in an American puzzle magazine in 1979. The global explosion came later still: The Times of London started printing it in 2004, and within a couple of years it was in newspapers everywhere, with the puzzle books and apps and competitions that followed.

Core techniques

Everything starts with elimination. If a digit already appears in a row, column or box, it can't appear again in that unit. Scan all three constraints at once for an empty cell, and often only one digit survives. That's a naked single, and it needs nothing more sophisticated than counting.

Once the naked singles run out, go looking for hidden singles: a digit that can only go in one cell within a row, column or box, even when that cell looks like it has several options. Past singles, you get pairs and triples, where candidates confined to the same cells within a unit let you strike those digits from every other cell in it.

Most easy and medium puzzles fall entirely to those three. Hard ones can ask for longer chains, but the principle doesn't change: constrain, eliminate, confirm.

Notes and perfect wins

Notes mode lets you pencil in candidate digits, the small numbers that might belong in a cell. Good players keep them meticulously: every time you place a digit, strip it from the notes of every cell in the same row, column and box. Do that and the puzzle stops being a memory exercise and becomes something you can just look at. Since a perfect win means no mistakes and no hints, notes discipline is the most reliable route to one.

Difficulty levels

Easy puzzles fall to naked and hidden singles almost entirely. Medium ones want pairs and the occasional bit of looking ahead. Hard ones need chains and deliberate candidate management, and they start you off with barely any naked singles at all.

All three use the same rules and the same grid. What changes is the number of givens and the techniques you need to resolve everything else. Best times are recorded separately per difficulty, so progress in each track means something on its own terms.

Want one difficulty on tap? Easy sudoku deals boards with around 50 givens, medium around 38, and hard digs down to 24. All unlimited, none of them touching your streak.

Tips for beginners

Start with whichever digit appears most often in the givens. If seven nines are already placed, the last two are easy. Work through the digits one at a time before you drop into open-ended scanning.

When you're stuck, switch to notes mode and mark every candidate for every empty cell. The pattern of candidates usually shows you a hidden single or pair that you couldn't see without it. And don't guess. If you can't find a deduction, you've missed one. Step back, check your notes are current, scan again.