Sudoku

The rules

Fill every empty square so that each row, each column and each box contains every digit exactly once (1–9 on the classic grid, 1–6 on the 6×6 Starter). Every puzzle here has exactly one solution, and it can always be reached by reasoning. If you find yourself guessing, a better move is waiting to be found.

Controls

• Tap a square, then a number. Turn Notes on to pencil in possibilities (keys: N).
• Keyboard: digits to fill, arrows to move, Backspace to erase, Ctrl/Cmd+Z to undo, H for a hint.
• Hint works in two steps: a nudge toward where to look, then, if you ask again, the full explanation.

The ladder of techniques

• Naked single Easy A square where only one digit survives.
• Hidden single Easy A digit with only one possible square left in a row, column or box.
• Locked candidates Medium A digit confined to one line inside a box (or one box inside a line) clears that digit from neighbouring squares.
• Naked and hidden pairs Medium Two squares that share two digits between them lock those digits in.
• Triples and X-Wing Hard The same idea on larger sets and across rows and columns.

When you finish, the puzzle tells you which of these it needed, and shows you the exact spot in your grid where the technique applies.