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How this Sudoku works

Fill the nine-by-nine grid so that every row, every column and every three-by-three box contains the digits 1 to 9 exactly once. Puzzles are generated fresh at Easy, Medium and Hard, each with a single valid solution, so you never need to guess.

Tap a cell, then a number. A notes mode lets you pencil in candidates, matching numbers and conflicts are highlighted, a hint button helps when you're stuck, and undo is one tap. A timer runs in the background, a completion message tells you when you're done, and your current puzzle is saved on your device so you can close the tab and come back.

Where Sudoku comes from

The modern puzzle was designed by American Howard Garns and published in 1979 in a Dell magazine as "Number Place". It became Sudoku in Japan, where the publisher Nikoli popularised it from 1984. It reached a worldwide audience in the 2000s after a newspaper in London began printing it daily.

Techniques that solve almost everything

1. Scan for singles. Pick a digit and see where it can still go in each box. If only one cell fits, that's your answer.
2. Look for cells with one candidate. Pencil in the possibilities and some cells will only have one.
3. Use pairs. If two cells in a row can only hold the same two numbers, no other cell in that row can use them.
4. Work the busiest rows first. The more digits already placed, the fewer options remain.
5. Never guess. If you're guessing, there's a deduction you haven't made yet.

What the levels mean

Difficulty in this game comes from how many numbers are given at the start. Easy begins with 42 filled cells, Medium with 34 and Hard with 28. Fewer clues mean longer chains of reasoning, not necessarily stranger techniques. Easy puzzles fall to scanning for singles, and Hard puzzles usually need pencil notes and pairs.

Beyond the basics

Once you're comfortable, two more ideas open up harder puzzles. A pointing pair happens when a digit's only possible cells inside a box all sit in one row, which removes that digit from the rest of the row. An X-wing uses two rows where a digit can only go in the same two columns, which clears it from those columns elsewhere. You'll rarely need them on Medium, but Hard puzzles may call for them.

Mistakes that cost the most time

• Guessing. A wrong guess can take many moves to show up and is miserable to undo. Use notes, or the undo button, to test an idea safely.
• Ignoring the boxes. Beginners check rows and columns and forget the three-by-three boxes.
• Stale notes. If you pencil in candidates, clear them as numbers are placed, or the notes will mislead you.
• Not using the highlights. Tap a number and the game shows you where that digit already sits, which is half of every deduction.

From a prompt

The puzzle generator and its checks for a single solution came from a written brief. Read the prompt on the game page.