How to play sudoku
Sudoku is a logic puzzle played on a 9x9 grid. The grid is split into nine rows, nine columns and nine 3x3 boxes, and some cells start with a number already in them.
Fill every empty cell so that each row, each column and each box contains the numbers 1 to 9 once each. That is the only rule. A proper sudoku has exactly one solution, and you can reach it by logic alone.
Finding your first numbers
Pick a number that already appears many times. For each box that lacks it, cross out in your head the rows and columns that already have it. If only one cell in the box is left, the number goes there.
Then look at rows, columns and boxes that are nearly full. If a row is missing two numbers, check the columns of its two empty cells: one of them will often already hold one of the two.
Pencil notes
When a cell could be one of several numbers, write them in small as notes. Notes are called candidates. As you place numbers, the candidates around them shrink, and a cell with one candidate left is solved.
Turn on Notes to write candidates by hand, or tap Fill notes to mark every legal candidate in every empty cell at once. Fill next writes in every cell that is down to a single number.
The buttons
Hint marks the next step on the board and explains it in a sentence. It never fills a number in by itself, but you can apply the step with one tap. Hints are unlimited.
Colour turns the number pad into nine paints, so you can mark cells while you follow a chain. Undo and Erase are under More. Restart takes the same puzzle back to its starting numbers, and New game deals a fresh one at the level you choose.
A wrong number is shown in red. There is no mistake limit, so a slip never ends the game. The timer can be paused, and it pauses by itself when you leave the tab.
Playing with a keyboard
Move with the arrow keys and type a number to fill the selected cell. Hold Shift while typing to switch between a number and a note. C opens the colours, A fills the next single numbers, Enter applies a hint, Backspace erases and Cmd or Ctrl with Z undoes.
Where to go next
Start with an easy puzzle. When easy grids fall without a hint, move up one level at a time. Each level page explains the technique that level adds.