What is Slitherlink?
Slitherlink is a puzzle where you draw lines between neighbouring dots on a grid until they form a single closed loop. A number from 0 to 3 in a cell says how many of that cell's four sides are part of the loop, and empty cells can have any number of sides. The puzzle is solved when every line belongs to one loop with no branches and no loose ends.
It is also known as Loop the Loop, Fences or simply the loop puzzle. The rules fit in two lines, yet switching between the numbers and the dots to pin down each line gives it the same deep deductive flavour as Sudoku and Kakuro, which is why it is one of the classic pencil puzzles.
| Rule | Meaning |
|---|---|
| Number | How many of the cell's four sides carry a line. 0 means none, 3 means three |
| Empty cell | Any number of sides may carry a line |
| Dot | Either 0 or 2 lines meet at a dot. One line is a dead end, three is a fork |
| Loop | All lines form one closed loop. A separate small loop is not allowed |
| × mark | A note that no line can go there. It does not affect the answer check |
Where should I start drawing?
The classic approach is to start where the lines are forced at a glance: a 3 next to a 0, a 3 in a corner, two 3s side by side. A handful of these opening patterns fills half the board on its own, and the rest follows from the rule that each dot has either 0 or 2 lines.
| Pattern | What to draw |
|---|---|
| 0 | All four sides are ×. A line passing a corner of a 0 must go straight past it |
| 3 next to a 0 | The three sides not shared with the 0 are lines, and lines continue outward from the two corners on the 0 side |
| 3 in a corner | Both sides touching the edge of the board are lines |
| 1 in a corner | Both sides touching the edge of the board are × |
| 2 in a corner | The two segments leading away from the corner along the border are lines |
| Adjacent 3 and 3 | The shared side and the two outer parallel sides are all lines, and the segments sticking out perpendicular from the ends of the middle line are × |
| Diagonal 3 and 3 | Each 3 gets its two sides facing away from the other, four lines in all |
| A line enters a corner of a 3 | The two sides opposite that corner are lines |
| Both outer segments at a corner of a 1 are × | The two sides of the 1 at that corner are × too |
Is there only one answer? How are boards made?
Yes. Every board is generated fresh when you open it, and only boards that the built in solver can reason through to a single answer are used. The solver confirms the puzzle can be solved by logic alone, then removes clues one at a time, keeping each removal only if the board still solves.
First a random blob of cells is grown and its outline becomes the answer loop. Holes in the blob or two cells touching only at a corner would split the loop or put four lines on one dot, so those shapes are blocked from the start. Then every cell gets its number and the clues are thinned out under the solver's watch. Easy keeps more than half of the numbers, Normal removes as many as logic allows, and Hard goes further with a backtracking solver that checks the answer is still unique. The whole process takes well under a second, so the next board is always ready.
What changes between difficulty levels?
Easy is 5×5 with plenty of numbers, Normal is 7×7, and Hard is 10×10 with far fewer numbers and longer chains of deduction. Bigger boards mean a longer loop, and wide empty areas demand that you look ahead so a small loop does not close by accident.
| Level | Board | Character |
|---|---|---|
| Easy | 5×5 | More than half the cells are numbered, so the opening patterns carry you to the end |
| Normal | 7×7 | About 40% of cells numbered. You pull distant lines together with the dot rule |
| Hard | 10×10 | Fewer numbers, and you sometimes test whether a small loop would close |
| Daily puzzle | 7×7 | One board per day, the same for everyone. Solving it counts toward your daily streak |
What help is there when I am stuck?
Hint draws one correct line in gold, and Check turns any line that is not in the answer, or any × sitting on an answer line, red. Undo reverts the last stroke, and a drag that drew several lines comes back as one step.
- Hint: if you have a wrong line, it erases that first; otherwise it draws one answer line that extends what you already have. The number of hints used is shown with your result.
- Check: when nothing is wrong it says so, which makes it a handy mid game sanity check.
- Number colours: a number fades when exactly the right number of sides is drawn and turns red when there is one too many. Fix red numbers on the spot.
How do I solve Slitherlink faster?
- Use × marks generously. Marking impossible gaps reveals dots with only one free gap left, and that gap must be a line.
- Watch the dots as closely as the numbers. A dot with one line in needs exactly one line out, and a dot with two lines rules out the rest.
- Beware of small loops. A loop that closes on its own makes the board wrong, so the gap that would close it is almost always a ×.
- 1s and 3s help each other. One line into a 1 makes its other three sides ×, and one × on a 3 makes its other three sides lines.
- Think inside and outside. The loop splits the board into an inside and an outside. Two cells on the same side share no line; cells on different sides must have one between them.
Controls
- Phone and tablet: tap between two dots once for a line, again for ×, and once more to clear. Drag without lifting to draw lines through every gap you pass.
- Mouse: left click cycles line → × → clear, right click toggles ×. Hold and drag to draw several lines at once.
- Keyboard: arrow keys move a cursor over the dots, Shift+arrow draws a line in that direction, Ctrl+arrow places an ×. Z undo, H hint, C check.
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