Kropki Sudoku (PuzzleMadness)

About Kropki Sudoku (PuzzleMadness)

Kropki Sudoku keeps the usual sudoku rules: each row, column and 3x3 box holds the digits 1 to 9 once. On top of that, dots sit on some of the borders between neighbouring cells. A white dot means the two digits differ by one, such as 4 and 5. A black dot means one digit is twice the other, such as 3 and 6. The pair 1 and 2 could take either colour. The missing dots matter as much as the printed ones: two neighbours with no dot between them are neither consecutive nor in a 1:2 ratio.

Because of the dots, the grids start with fewer given digits than a classic sudoku of the same difficulty. Today's Medium grid, for example, starts with 26 given digits and dots spread across every box.

PuzzleMadness is a free puzzle website run by Dewi, a software developer who started out making games for the PlayStation 2, Nintendo Wii and DS and now builds the site as a hobby. It carries dozens of puzzle types, from classic sudoku to newer logic grids, and each one gets its own dated puzzle every day at each difficulty. Kropki Sudoku comes in Easy, Medium, Hard and Tough versions plus a Mixed page, and the same day's Kropki Sudoku puzzle is shared by everyone who opens it.

The site checks your grid as you go and tells you when it is finished; it does not publish solutions. Each completed puzzle earns a score, and scores feed a monthly and an all-time Kropki Sudoku leaderboard, where only your best attempt at a puzzle counts. Older days stay in the archive, so a missed puzzle can still be played later and still counts toward the current month. Playing needs no account; a free registration gives you a username on the leaderboards and a cloud save. The pages carry adverts.

How to play Kropki Sudoku (PuzzleMadness)

1. Open today's Kropki Sudoku page on PuzzleMadness and choose a difficulty from the menu above the grid.
2. Click a cell and enter a digit with the number buttons above and below the grid or with the keyboard; the eraser button clears a cell.
3. Fill every row, column and 3x3 box with the digits 1 to 9, each once.
4. Where a white dot joins two cells, make their digits consecutive; where a black dot joins them, make one twice the other.
5. Where there is no dot between two neighbours, their digits must be neither consecutive nor double.
6. Turn on pencil mode (the pencil button, or Ctrl, Shift or Alt with a number key) to jot candidate digits in a cell.
7. The site tells you when the grid is complete and adds your score; registered players appear on the leaderboards.

Screenshots

Tips

• A black dot can only join 1-2, 2-4, 3-6 or 4-8, so cells on a black dot never hold 5, 7 or 9.
• Chains of white dots give runs of consecutive digits; three cells in a white chain must hold three numbers in a row.
• Use the no-dot rule: a cell next to a 5 with no dot between them cannot be 4 or 6.
• Missed a day? Earlier puzzles are in the archive, and finishing one still counts toward this month's leaderboard.

Questions