1 of 4
Tap every digit that could go on this green line.
Select all that apply
Level 3 · Lines
How to solve German whispers
Neighbors on a green line differ by at least 5. This one has a famous trick.
A German whispers line is a green line through the grid. Any two cells next to each other on the line must differ by at least 5. Digits can repeat on the line when sudoku rules allow it.
The rule splits the digits into low (1 to 4) and high (6 to 9), and 5 is left out entirely. Once you know whether one cell is low or high, the whole line falls into a pattern.
How setters write it
The same rule, worded the ways you’ll see it on puzzles.
- Adjacent digits on a green line differ by at least 5.
- German whispers.
- Digits along a green line must have a difference of at least 5.
Worth memorizing
- 5 can never go on a green line. No digit from 1 to 9 is 5 away from 5. Every solver learns this first.
- 4 can only sit next to 9, and 6 only next to 1.
- Digits swing low (1–4) and high (6–9) along the line, so every other cell has the same type.
Solving tips
5 never goes on the line
No digit from 1 to 9 is 5 or more away from 5. Remove 5 from every cell on a green line before you do anything else.
4 and 6 are nearly placed
4 can only sit next to 9, and 6 can only sit next to 1. A 4 can't sit in the middle of a straight stretch of line, since it would need two 9s in the same row or column.
Lines alternate high and low
Every cell on the line is low (1 to 4) or high (6 to 9), and neighbors switch type. Color one cell and every other cell along the line gets the same color.
Squeeze the middle cells
A low cell between two high neighbors in the same box needs two different digits at least 5 above it, so it is 3 or lower. A high cell between two lows in one box is 7 or higher.
A worked example: a 3-cell green line inside one box
A green line visits three cells of one box and bends at the middle cell, so the middle cell has two neighbors on the line. The box already has a 1, a 7 and a 9 placed away from the line.
- Remove 5 from all three cells, as always. Then look at the middle cell. Its two neighbors are in the same box, so they must be two different digits, both at least 5 away from it.
- If the middle is low, it needs two different digits at least 5 above it. A 4 only has 9, so 4 fails. The middle can be 1, 2 or 3 if low. By the same logic it can be 7, 8 or 9 if high, and 6 fails.
- Now use the box. 1, 7 and 9 are already placed. A low middle of 2 would need 7 and 8, and 7 is taken. A low middle of 3 would need 8 and 9, and 9 is taken. So the middle can't be low.
- A high middle of 8 needs two lows at most 3, and with 1 gone that leaves 2 and 3. That works. The middle is 8, and the two ends are 2 and 3 in some order.
- Without the 7 in the box, the line could also have been 7, 2, 8 with 2 in the middle. Either way it held a 2 and an 8. That kind of shared result is worth noting before you have the full answer.
Practice
2 of 4
What goes next to the 4?
3 of 4
What goes next to the 6?
4 of 4
Tap every digit that could go next to the 3.
Select all that apply
Reference table
| Digit | Can sit next to | Options |
|---|---|---|
| 1 | 6, 7, 8 or 9 | 4 |
| 2 | 7, 8 or 9 | 3 |
| 3 | 8 or 9 | 2 |
| 4 | 9 | 1 |
| 5 | none (can't go on the line) | 0 |
| 6 | 1 | 1 |
| 7 | 1 or 2 | 2 |
| 8 | 1, 2 or 3 | 3 |
| 9 | 1, 2, 3 or 4 | 4 |
Going deeper
Counting highs and lows in a box
Every box has four low digits (1 to 4), four high digits (6 to 9) and one 5. The 5 can't go on the line. So if a green line covers 8 cells of a box, the ninth cell of that box is the 5.
Color the line in two colors as you go. Cells of one color are all low or all high. A single color can't cover more than four cells in any row, column or box, since there are only four digits of each type. If one color reaches four cells in a box, those cells hold all four digits of that type.
When 4 or 6 can sit in the middle
A 4 needs a 9 on every side along the line. When the line runs straight along a row or column, both neighbors share that row or column, so they can't both be 9. The same is true when both neighbors sit in the same box.
At a bend, or on a diagonal stretch, the two neighbors are in different rows and different columns. If they are also in different boxes, both can be 9. So a 4 (with a 9 on each side) or a 6 (with a 1 on each side) can sit where the line turns at a box border. Check the shape of the line before ruling them out.
End cells and short lines
An end cell has only one neighbor on the line, so it has the most freedom. A 4 or 6 on a straight line usually ends up at an end. When an end cell could be 4, look at its one neighbor. If that neighbor can't be 9, the 4 is gone.
Short lines work the same way. A 2-cell green line is just one pair, and the table on this page lists every option. A 3-cell straight line has one middle cell, so its middle can only be 1, 2, 3, 7, 8 or 9.
Common mistakes
Applying the rule to cells that only touch
The gap of 5 only applies to cells joined by the line. Two cells can sit side by side in the grid without being line neighbors, for example where a line folds back on itself. Those cells have no whispers rule between them.
Thinking the gap must be exactly 5
The rule is at least 5. So 1 and 9 are fine, even though they are 8 apart. Only 4 and 6 have a single partner, which is why they feel like exact matches.
Forgetting the line can repeat digits
Two cells far apart on the line can hold the same digit when they don't share a row, column or box. Don't eliminate a 9 from a cell just because another 9 sits further along the same line.
In mixed-rule puzzles
German whispers pair up with other rules in narrow ways. A black Kropki dot can never sit on a whispers pair, because the 1:2 pairs (1 and 2, 2 and 4, 3 and 6, 4 and 8) are all less than 5 apart. A white dot can't either. An X on a whispers pair must be 1 and 9 or 2 and 8, since 3 and 7 and 4 and 6 are too close. On a thermo, a whispers line can share at most two cells, because three rising digits can't swing low, high, low. With a renban line, two renban neighbors on a whispers line need a renban of at least 6 cells to have a gap of 5.
What the video shows
- Neighbors on a green line differ by at least 5.
- 2 and 7 are 5 apart.
- 7 then 4 is only 3 apart.
- Digits swing low, high, low, high. 5 can never go on a whisper.
Questions solvers ask
Can digits repeat on a German whispers line?
Yes, as long as the two cells don't share a row, column or box. The line only limits cells that are next to each other on it.
Why can't 5 go on a German whispers line?
A neighbor of 5 would need to be 0 or 10, and neither is a sudoku digit.
What is the difference between German and Dutch whispers?
German whispers need neighbors at least 5 apart. Dutch whispers need at least 4 apart, which lets a 5 sit on the line next to a 1 or a 9.
What color is a German whispers line?
It is usually green. Check the rules text, since some puzzles use other colors.
What can go next to a 3 on German whispers?
Only 8 or 9. Both are at least 5 above 3, and nothing below 3 is far enough away.
Where does the 5 go in a box with a German whispers line?
In a cell off the line. If the line covers 8 cells of the box, the last cell must be the 5.
Do German whispers work on diagonal steps?
Yes. The rule applies to any two cells next to each other on the line, including where the line moves diagonally.
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