Slitherlink
Play Slitherlink (Loop the Loop) puzzles online. Draw a single closed loop along the grid edges so each numbered cell touches exactly that many line segments. 5×5 to 10×10.
The rules
Slitherlink — also sold as Loop the Loop, Fences, or simply Number Loop — gives you a grid of dots with numbers hiding in some of the cells. The goal is to draw a single closed loop that runs only along the dotted lattice.
Two iron rules:
- Each numbered cell must be surrounded by exactly that many line segments (out of the four edges around it).
- The line you draw must form one single loop — no branches, no crossings, no separate loops.
Click an edge to draw a line; click it again to clear it. That single toggle is all you need to trace the loop.
Here a single loop wraps the top-left 2×2 block. Read it clue by clue:
- Each 2 sits in a corner of the loop — exactly two of its edges are line segments.
- Each 1 is touched by the loop on a single side.
- The 0 is completely outside — none of its edges are used.
What each clue means
| Number | Edges around the cell |
|---|---|
| 0 | None — the loop avoids this cell entirely. All four edges get ×. |
| 1 | Exactly one of the four edges is a line — the rest get ×. |
| 2 | Two opposite or two adjacent edges are part of the loop. The trickiest clue — the loop turns inside the cell or passes straight through. |
| 3 | Three of the four edges are lines — the loop hugs the cell on three sides. |
Solving techniques
- Start with the corners. A corner cell only has two edges that can join. A corner clue of 1 forces both grid-edge sides to be
×; a corner 3 forces both grid-edge sides to be a line. - Look for 0s and 3s next to each other. A 3 against a 0 forces the three edges of the 3 away from the 0. Adjacent 3s force a known line between them plus two parallel lines on the outside.
- Track lattice points. Every dot must have either 0 or 2 lines meeting at it. A dot with one line already needs exactly one more — and never more than two.
- Avoid premature small loops. Sometimes a step would close a tiny loop in one corner of the grid while leaving line segments untouched elsewhere. That step must be wrong — the final loop has to be a single loop, so any path that closes early is invalid.
- Parity along a row. A horizontal line drawn across the grid crosses the loop an even number of times. This lets you check whether a row of edges is consistent with a single enclosed region.
About the puzzle
Slitherlink was first published by Nikoli — Tokyo's famous puzzle magazine — in 1989, the same year it released Shikaku. The English export markets sometimes call it Fences or Loop the Loop. Unlike Sudoku, every move is geometric, and unlike Shikaku the medium isn't rectangles but the boundary between “inside” and “outside” cells. A well-formed Slitherlink has exactly one solution reachable by deduction alone — guessing is never needed.