Sudoku Classic

On a computer, click a cell or move around the grid with the arrow keys (or W, A, S, D). Type 1–9 to fill the selected cell, and Backspace, Delete or 0 to clear it. Press N to switch to notes, U to undo, H for a hint and C to turn auto-check on or off.

On a phone or tablet, tap a cell, then tap a digit on the keypad below the grid. Use the Undo, Erase, Notes, Hint and Check buttons above the keypad. With Check on, any digit that doesn’t match the solution turns red straight away; with it off, only digits that clash with another in the same row, column or box are marked. The small number in the corner of each key shows how many of that digit are still missing; a key fades once all nine are placed.

Choose Easy, Medium, Hard or Expert from the menu at the top, press New for a fresh puzzle, or press Daily for today’s shared puzzle. The timer, your best time for the chosen level and your mistakes are shown above the grid.

1. Each row must contain the digits 1 to 9, with no repeats.
2. Each column must contain the digits 1 to 9, with no repeats.
3. Each of the nine 3×3 boxes must contain the digits 1 to 9, with no repeats.
4. The starting digits (shown on shaded squares) are fixed and cannot be changed.
5. Every puzzle has exactly one solution. You win as soon as all 81 cells match it.
6. Your score is your solving time. Lower is better, and each difficulty keeps its own best.

Start with singles. Look for a cell where eight different digits already appear across its row, column and box — only one digit is left for it. Selecting a filled cell highlights every matching digit, which makes this quick to spot.

Scan for hidden singles. Pick a digit and a box, and ask where that digit can go inside the box. If the other cells are all blocked by the same digit in nearby rows and columns, there is only one place left, even if that cell could hold other digits in theory.

Use notes once the easy moves run out. Pencil in the candidates for a box or a row. When two cells in a unit share exactly the same two candidates, those two digits belong to those cells, so you can erase them everywhere else in that unit.

Look at box-line overlaps. If every place a digit can go in a box lies in one row, that digit can’t appear anywhere else in that row outside the box. The same works the other way round, from a row or column into a box.

Work on the most constrained areas. Rows, columns and boxes with the most digits filled in give the fastest progress. Come back to the open areas later, when they have fewer options.

Treat hints as a teacher. A hint fills the selected cell, or the cell with the fewest possibilities. Afterwards, try to work out why that digit had to go there — that’s how you get faster.

Puzzles that ask you to arrange numbers in a grid go back centuries, to magic squares and to Leonhard Euler’s eighteenth-century work on Latin squares. In the late 1800s, French newspapers printed number puzzles that came close to the modern form. The version we know today first appeared in 1979 in an American puzzle magazine under the title “Number Place”, reportedly created by retired architect Howard Garns. It spread to Japan in the 1980s, where it got the name sudoku, a shortened phrase meaning roughly “the digits must remain single”. Japanese editors also added the convention of symmetrical clue patterns. A worldwide craze followed in 2004 and 2005, when British newspapers began running a daily grid. Sudoku Classic is ArcadeWren’s own take, with its own grid design and puzzles made fresh in your browser.

• Daily puzzle. One medium grid per day, identical for every player, so you can race friends over the same puzzle.
• Expert grids. Sparse puzzles that need careful note-taking and chains of eliminations.
• No-notes challenge. Solve a Medium grid without writing a single pencil mark, holding the candidates in your head.
• Killer and cage variants. Versions that add sums over groups of cells, which ArcadeWren will offer separately.