German Whispers Sudoku: rules and tips

German whispers sudoku draws green lines through the grid. Along a line, any two neighbouring digits must differ by at least 5.

That single rule has surprisingly strong consequences: some digits can’t be on a line at all, others are almost trapped, and every line alternates between high and low.

The rules

• Normal sudoku rules apply: 1 to 9 once in every row, column and 3×3 box.
• Any two cells next to each other along a green line differ by at least 5.
• Lines can bend and step diagonally, and they can cross box borders.

How to start

Five is out

A 5 would need a neighbour of 0 or 10, so 5 never appears on a whisper line.

Lines alternate high and low

Two low digits (1–4) differ by at most 3, and so do two high digits (6–9). So along a line the digits must alternate low, high, low, high. Once you know the polarity of one cell you know the whole line.

Fours and sixes are fussy

A 4 can only sit next to 9, and a 6 only next to 1. So a 4 or 6 in the middle of a line needs the same digit on both sides, which is impossible if those two neighbours share a row, column or box.

Count the lows in a box

Each box has only four low and four high digits. If whisper lines force five cells of a box to be low, something is wrong; if they force four, the remaining cells of the box must be 5 or high.

Common mistakes

• Putting a 5 on a line.
• Forgetting that a difference of exactly 5 is allowed, such as 1 and 6 or 4 and 9.
• Missing the diagonal steps on a bent line.

How the ranks scale difficulty

Every puzzle type on Hoot’s Puzzles comes in four ranks, named after the stages of a mathematical idea. Lemma is the warm-up: about a dozen more digits are given than logic strictly needs, and singles plus the variant’s own rule will carry you through. Proposition needs a real idea or two, typically pointing and claiming. Theorem is near-minimal, so almost every clue is load-bearing and you may need pairs. Conjecture gives the fewest digits and sparser rule clues: expect to think hard.

Higher ranks use fewer lines, around six at Lemma down to about three at Conjecture. Lines here run five to ten cells.

There are 99,999 puzzles per rank, each with its own number, so you can replay one or share it. Every type also has a 30-puzzle collection that climbs from Lemma to Conjecture. Every puzzle is checked to have exactly one solution that you can reach by logic alone, with no guessing.

Keep exploring

• Renban SudokuPurple lines hold runs of consecutive digits, in any order.
• Thermo SudokuDigits strictly increase from bulb to tip. Count the cells, bound the ends.
• Kropki SudokuWhite dots for consecutive pairs, black for a 1:2 ratio. Missing dots matter.
• Solving techniquesSingles, pairs, cage arithmetic, line logic and Latin squares, all in one guide.
• All puzzle guidesEvery puzzle type on Hoot’s Puzzles, with rules and tips.