Classic sudoku is a logic puzzle played on a 9×9 grid. The grid is split into nine 3×3 boxes. Some cells already hold a digit from 1 to 9. The task is to fill the rest so that every row, every column and every box contains each digit exactly once.

A well-made puzzle has exactly one solution, and you can reach it by reasoning alone. You never need to guess. That is what makes sudoku different from many other number games: each move follows from the ones before it, and a wrong move shows up later as a contradiction you can trace back.

The grid idea is much older than the game. Mathematicians studied Latin squares, arrangements of symbols in a grid with no repeats in any row or column, long before puzzles used them. Leonhard Euler wrote about such squares in the eighteenth century.

The puzzle in its modern form appeared in 1979. An American puzzle designer, Howard Garns, published a version called "Number Place" in a Dell puzzle magazine. Japanese publisher Nikoli adopted the idea in the mid-1980s and gave it the name Sudoku, a short form of "su-ji wa dokushin ni kagiru", roughly "the digits must stay single". The puzzle spread through Japan through the 1980s and reached newspapers and books around the world in the early 2000s. British newspapers helped make it a daily habit for many readers.

So the game is less than fifty years old, but the mathematics behind it has been studied for much longer.

Most generators start from a full, valid grid and remove digits one at a time while keeping a single solution. The number of starting digits that remain decides how hard the puzzle is. Fewer clues mean more candidates for each empty cell, and more steps to reach the answer.

This is why two puzzles with the same number of clues can feel very different. The position of the clues matters as much as their count.

Classic mode has five levels, and each one asks for a different kind of thinking:

• Easy (Лёгкий) has the most starting digits. Most empty cells have only one possible digit once you look at their row, column and box. A good place to learn how the rules work.
• Medium (Средний) starts to ask for pairs. You note two candidates in two cells of one unit, and that pair can rule out digits elsewhere in that unit.
• Hard (Сложный) requires looking for a digit that fits in only one cell of a box, even when that cell has other candidates.
• Expert (Экспертный) has few starting digits. Progress often depends on a chain: a deduction in one place unlocks a deduction in another.
• Master (Мастерский) has the fewest clues and the longest chains. Expect to spend time on a single step and to use notes heavily.

You can move between levels at any time. Many players keep Easy for warm-up and go up when the puzzles start to feel routine.

Most solving methods come down to a few patterns. Learn these first:

• Single candidate. A cell where only one digit remains possible.
• Hidden single. A digit that has only one place left in a row, column or box, even if that cell still shows other candidates.
• Naked pair. Two cells in the same unit that share exactly the same two candidates. Those two digits cannot appear anywhere else in that unit.
• Pointing pair. When the candidates for one digit in a box all sit on one line, that digit is removed from the rest of the line outside the box.

You do not need the names to use the methods. The names are useful when you read about strategy, which is why they are listed here.

Choose an empty cell, then a digit. Wrong digits count as mistakes, and the game tracks them for you. Notes let you mark the digits a cell could still hold, so you do not have to keep them in your head. The eraser clears a cell. Undo takes back the last correct move together with its points. A mistake or a hint cannot be undone, because it has already cost you points.

A game with more than three mistakes or three hints keeps going, but it stops counting toward the rankings. This keeps the leaderboard fair while still letting you finish a puzzle you have started. The Rules page explains the points in full.

Sudoku is often described as "good for the brain", and that claim is only partly supported. Puzzles of this kind ask for sustained attention, working memory and the habit of testing an idea before committing to it. Those are real skills, and practising them is pleasant.

Research on whether puzzles protect against memory decline or dementia has not produced clear, consistent results. A daily puzzle is a reasonable habit and a good way to slow down, but it is not a treatment or a prevention plan. If you are worried about memory or concentration, talk to a doctor.

What a regular habit reliably gives most people is focus. A puzzle takes five or ten minutes, and it forces attention onto one problem at a time, away from the feed.

Most beginners get stuck for the same three reasons: they guess instead of deducing, they use notes too early or not at all, and they scan cells one by one instead of looking for digits. The article 3 mistakes almost every sudoku beginner makes covers each of them with examples.

A simple routine helps. Before each move, ask which digit is the easiest to place and where it can go. When you have no move, add notes to one unit at a time, not to the whole grid.

Both formats work. Paper slows you down in a useful way, and a screen catches your errors and makes trying candidates cheap. Many players use both. The paper or screen guide compares them, and the printable page produces ready-to-print puzzles in A4 or US Letter.