Slitherlink

Slitherlink is one of Nikoli's original loop puzzles and first appeared in Puzzle Communication Nikoli issue 26 in June 1989. The grid gives you dots, numbered cells, and one job: draw a single closed loop. A clue tells you how many sides of that cell belong to the loop. The puzzle trains local counting and global shape control at the same time. Short segments must eventually become one loop, with no branches and no separate cycles. Start drawing edges and closing gaps.

1. Look at each numbered cell. The number tells how many of its four sides must be loop edges.
2. Draw confirmed edges between neighboring dots.
3. Mark confirmed non-edges when a side cannot be used.
4. Keep all line segments connected into one final loop.
5. Avoid branches, loose ends, and small separate loops.
6. Finish when every clue is satisfied and the loop closes once.

• A 0 cell has no loop edges around it.
• A 1 cell has exactly one loop edge.
• A 2 cell has exactly two loop edges.
• A 3 cell has exactly three loop edges.
• The final line must form one continuous loop.
• The loop cannot cross, branch, or end early.

• Start with 0 clues. Mark all four sides as unused, then use those marks to solve nearby clues.
• A 3 beside a 0 forces the three sides away from the 0 in many common positions.
• Never let a dot have three line segments. Each loop dot can use only two edges.
• Avoid closing a small loop before all clues connect to it.
• Use corner pairs. Adjacent 3s near an edge often force long outside segments.

• Bridges: Also builds a connected network, but uses numbered islands and straight bridges.
• Nurikabe: Uses grid connectivity with shaded sea cells instead of loop edges.
• Masyu: A loop puzzle that uses black and white pearl clues.

• The numbers 0 through 3 create many strong patterns.
• The single-loop rule adds whole-board tension.
• Progress is visible as the line grows.
• Solvers can learn reusable patterns without memorizing long formulas.
• Extra Large grids test endurance and whole-board awareness with complex loop paths.

Play Slitherlink online for free. Draw one loop, satisfy every clue, and use non-edge marks to prevent branches and early cycles. Small grids are best for learning the core patterns.

Slitherlink is a compact loop puzzle with rich logic. Count edges, block impossible sides, and keep every segment part of one final loop.

• Nikoli Slitherlink Rules
• Slitherlink Puzzle Background