German Whisper Line Sudoku (PuzzleMadness)

Every row, column and 3x3 box holds 1 to 9 once each, as usual. Along each German Whisper line, each pair of neighbouring cells must hold digits at least 5 apart. That makes 5 impossible anywhere on a line, because no digit from 1 to 9 is five or more away from it. The rest split into low digits (1 to 4) and high digits (6 to 9), and moving along a line the digits alternate between the two halves.

The alternation is the main tool: once you know whether one cell of a line is high or low, every cell on the line follows. 4 and 6 are tight too, since a 4 can only sit beside a 9 and a 6 beside a 1. Repeats are allowed along a line when the sudoku rules permit, as in the site's example, which runs 9, 3, 8, 3, 8.

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. German Whisper Line Sudoku comes in Easy, Medium, Hard and Tough versions plus a Mixed page, and the same day's German Whisper Line 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 German Whisper Line 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.