Classic Grid
Standard dot grid with 0–3 clues.
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Overview 
Slitherlink
1 player · 5-20 min per session
Draw a single continuous loop along the grid lines so that each numbered cell has exactly that many of its sides used by the loop.
Puzzle
Goal & Core Rules
Draw a single continuous loop along the grid lines so that each numbered cell has exactly that many of its sides used by the loop.
- Connect neighboring vertices with edges to form one closed loop.
- The loop cannot branch, leave loose ends, or split into multiple separate loops.
- A number tells how many bordering edges around that face belong to the loop.
- All numbered faces must be satisfied.
Different Sizes
Small quick loops or large grids for long deductions.
Controls
Mouse
- Click an edge: apply or clear the current input mode
- Right click: apply the opposite action from the current mode
- Use the menu or the control panel buttons to switch between line mode and block mode
Keyboard
- There is currently no dedicated keyboard control scheme.
Touch
- Use the bottom/right panel to choose line mode or block mode
- Tap an edge to apply or clear the selected mode
- Use pinch zoom and scrolling to inspect larger boards
Beginner Tips
- Start with 0s and 3s—they immediately forbid or force edges around a cell.
- Mark impossible edges with X to avoid accidental branching later.
- Follow the loop rule: any vertex can have degree 0 or 2—never 1 or 3.
Advanced Tips
- Avoid premature small loops unless you’re sure the entire loop is complete.
- Use parity/region reasoning: a loop must enter/leave areas in balanced ways on many grids.
- When you force an edge, immediately propagate vertex constraints two steps out.
Origins & History
Slitherlink is a Nikoli logic puzzle that first appeared in Puzzle Communication Nikoli issue #26 (June 1989). It also became known internationally under many alternate names such as Fences and Loop the Loop.
Timeline
- 1989 First appeared in Puzzle Communication Nikoli (issue #26, June 1989).
FAQ
Can the loop touch itself?
It can touch at corners, but it cannot cross or branch.
What do numbers 0 and 3 mean?
0 forbids all four sides; 3 forces three of the four sides to be in the loop.
What if I create two separate loops?
That violates the ‘single loop’ rule—merge them or undo the closure.