Rules

Sudoku rules

The board is a 9x9 grid split into nine 3x3 blocks. Fill every empty cell so each symbol (a digit or a flag, depending on the mode) appears exactly once in every row, every column, and every 3x3 block. Some cells are already filled in at the start - those are your given clues (Killer works a bit differently, see below).

Three modes

• Flags. Instead of the digits 1 through 9, the board holds the flags of nine random countries out of the full list of 194. The logic is identical to regular sudoku - you're just looking for, say, Kenya's flag instead of the digit 5. The Classic View button switches the display between flags and digits at any time without changing the game itself. One thing worth knowing: whichever view you spend most of the game in decides which mode the game counts toward.

• Classic. Regular sudoku with the digits 1 through 9, no flags. The rules, difficulty, and scoring are exactly the same as Flags, just without countries on the board.

• Killer. The board starts with some given digits, just like Classic and Flags, but usually fewer of them - and the higher the difficulty, the fewer there are, down to almost none at the hardest level. What replaces the missing clues is a set of cages: groups of cells marked with a dashed border, each showing its sum in the corner. Every digit inside a cage must add up to that number, and no digit can repeat within the same cage. The usual sudoku rules (row, column, 3x3 block) still apply on top of that.

Difficulty levels

The same 5 levels run through all three modes, from easiest to hardest - they're just named differently in Flags.

The higher the level, the fewer starting clues you get, and the bigger the score multiplier for every correct move - from x1 on level 1 up to x5 on level 5.

Buttons on the board

• Undo. Takes back your last move. A correct move is fully undone. A mistake or a used hint can't be undone, though: the cell goes back to empty, but the mistake/hint count and the penalty you already took stay.

• Eraser. Clears whatever you entered in the selected cell (a digit or notes), with no penalty.

• Notes. Notes mode lets you jot down several candidate digits in small print without committing to one. Handy for keeping options in view without risking a mistake.

• Hint. Reveals the correct answer for the selected cell. The first 3 hints in a game are free and still count toward the leaderboard. From the 4th on: Free watches a short ad before it's revealed, Premium sees it right away, but the game stops counting toward the flag standings and the player leaderboard.

• New game. Starts a fresh game at the selected difficulty. Free watches a short ad (about 30 seconds) before it starts, at most once every 5 minutes; Premium starts right away, no ad.

Mistakes and the leaderboard

A game only counts toward the flag standings and the player leaderboard while it has 3 mistakes or fewer, and 3 hints or fewer. Once you hit 3 mistakes (or use a 4th hint), you can still finish and solve the puzzle, but its points no longer count toward either leaderboard - Free sees an ad at that point, Premium gets a short notice, and the game goes on as normal either way.

How scoring works

Every correctly placed symbol earns points, based on the difficulty level (a x1 to x5 multiplier), how long your current streak of correct moves is, how fast you make the move, and any row, column, or 3x3 block the move completes. A mistake or a used hint costs points. Solving the whole puzzle adds a completion bonus, a bonus for each of the three mistakes you didn't use, and a speed bonus if you beat the level's target time.

In Flags mode, points from every move and the completion bonus don't go to just one flag - they're split across all nine flags on the board. More on that on the home page.

Free vs Premium

Printable sudoku works the same way: Free sees an ad before the PDF downloads, Premium doesn't.