Quadruple Sudoku (PuzzleMadness)

About Quadruple Sudoku (PuzzleMadness)

All the usual sudoku rules apply, with 1 to 9 once in every row, column and 3x3 box. The extra clues are small circles centred on the point where four cells touch. The digits printed in a circle must all be found among those four cells. A circle may show fewer than four digits, in which case the remaining cells are free, and it can show the same digit twice, meaning that digit appears in two of the four cells (which then have to lie diagonally from each other, since the other pairs share a row or column).

A circle with four digits fixes the contents of its 2x2 block entirely, leaving only the order to work out, while a circle with one or two digits is a softer hint that combines with the rows and columns around it. Circles that sit on a box border spread their digits across two or four boxes.

The grid works like every sudoku on PuzzleMadness: number buttons above and below it, an eraser, and a pencil mode for candidate digits. On a keyboard, the number keys enter digits, the arrow keys move between cells, Ctrl, Shift or Alt with a number adds a pencil mark, and Delete or Backspace clears a cell.

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. Quadruple Sudoku comes in Easy, Medium, Hard and Tough versions plus a Mixed page, and the same day's Quadruple 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 Quadruple 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 Quadruple Sudoku (PuzzleMadness)

1. Open today's Quadruple 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. For each small circle, place every digit it shows in one of the four cells around it; a digit shown twice goes in two of those cells.
5. Turn on pencil mode (the pencil button, or Ctrl, Shift or Alt with a number key) to jot candidate digits in a cell.
6. The site tells you when the grid is complete and adds your score; registered players appear on the leaderboards.

Tips

• A circle with a repeated digit puts that digit in two diagonal cells of its 2x2 block.
• A four-digit circle tells you exactly which digits the block holds, so pencil those four into all four cells and eliminate from there.
• Check circles against the rows and columns: a digit already placed in a row rules it out of the circle's cells in that row.
• Missed a day? Earlier puzzles are in the archive, and finishing one still counts toward this month's leaderboard.

Questions