Slitherlink

How to Play Slitherlink

Slitherlink is a closed-loop deduction puzzle played on a lattice of dots. Somewhere hidden in that lattice is a single unbroken loop, and your job is to draw it using nothing but the numbers scattered across the grid. Each number sits inside a cell and tells you exactly how many of that cell's four borders belong to the finished loop — a cell marked 2 has exactly two of its edges on the loop, a cell marked 0 has none of them, and a blank cell could have any count at all. Click or tap an edge to cycle it from undecided to included (a bright line) to excluded (a small mark), and use every clue in the grid to narrow down which edges must be which until a single continuous, non-branching loop remains.

The loop follows three hard rules: it must be a single closed curve with no gaps, it can never branch (no dot may have three or more included edges meeting at it), and it can never touch or cross itself. Because of those rules, deciding one edge often forces several others — mark a dot with two included edges already, and every other edge touching that dot must be excluded, because a third would create an illegal branch. That single piece of logic, applied over and over across the board, is what turns a scatter of numbers into a fully-traced constellation.

Tips and Strategy

• Start with the extremes. A cell marked 0 means all four of its edges are excluded immediately — mark them and move on. A cell marked 3 means exactly one edge is excluded, which is nearly as constraining.
• Corner cells marked 3 are a gift. A corner cell only has two edges touching the outside of the grid; a 3 there forces both of those outer edges included immediately, since a corner has at most 2 "internal" neighbours to exclude from.
• Watch every dot's degree. Any dot touching exactly two included edges already has its fate sealed — every other edge at that dot must be excluded. This single check catches more forced moves than any other technique.
• Two adjacent 3s force a shared edge. When two 3-clue cells sit side by side, the edge between them is included in every valid solution — a pattern worth memorizing on sight.
• Use the excluded mark, not just the line. Marking an edge excluded is just as informative as marking one included — it is what lets the degree-counting technique above actually work. Players who skip marking excluded edges solve far more slowly.
• When stuck, scan for the last piece. Near the end of a puzzle, most cells are already satisfied. The remaining undecided edges usually have only one legal value once you check which nearby dots still need a second included edge to close the loop.

What Makes Slitherlink Different

Most grid logic puzzles — Sudoku, Kakuro, Nonograms — ask you to fill cells with a value. Slitherlink asks something structurally different: it asks you to decide, for every edge in a lattice, whether that edge exists or not, subject to a single global shape constraint (exactly one simple closed loop). That shift from "what value goes here" to "does this connection exist" is what gives Slitherlink its distinctive rhythm. A single deduction rarely finishes a region on its own; instead, small local certainties (this edge is excluded because that dot already has two neighbours) ripple outward and eventually collide with a numbered cell on the far side of the board, which is exactly the moment a Slitherlink puzzle rewards patient, edge-by-edge tracking over guesswork.

The excluded-edge mark is the detail that separates a real Slitherlink implementation from a shallow copy. Many casual re-implementations only let you draw the line, which quietly removes half the puzzle's logic — the "this edge is definitely NOT part of the loop" deduction is just as load-bearing as the affirmative one, and without a fast way to record it, most of the harder puzzles become effectively unsolvable by hand. Every puzzle here is generated from a real closed loop and then stripped of clues only while a solver can still prove that loop is the single valid answer, so nothing you're asked to trace was left ambiguous by accident.

A Brief History of Slitherlink

Slitherlink was created by the Japanese puzzle publisher Nikoli — the same company behind Sudoku's modern popularization — and first appeared in a Nikoli puzzle magazine in 1989 under the name "Loop the Loop" before settling into its more common Japanese name, Suriza ("numbered loop"). It belongs to the same family of pencil-and-paper logic puzzles as Nurikabe and Masyu, all built around drawing a single connected shape on a grid rather than filling cells with symbols. Like most Nikoli puzzles, Slitherlink is defined entirely by its ruleset rather than any specific grid size, which is why it scales cleanly from a tiny 5×5 warm-up to expert-level boards well over 20×20 without ever changing its core logic.