Slitherlink
Draw one closed loop that obeys every number.
How to play Slitherlink
Connect the dots with horizontal and vertical segments to form a single continuous loop that never crosses or branches. Each numbered cell tells you exactly how many of its four sides the loop must use; empty cells can have any number. Use those clues to deduce the one valid loop, as fast as you can.
Mark what the loop cannot use
The single habit that turns Slitherlink from baffling to solvable is marking the edges the loop does not use, not just the ones it does. Every gap between two dots is either part of the loop or ruled out, and the ruled-out edges carry just as much information. A cell with a 0 in it is the perfect example: none of its four sides are used, so you cross out all four at once, and those little marks then squeeze every neighbouring number. Solvers who only draw lines and never mark the dead edges are throwing away half the puzzle.
The dot rule runs everything
The loop is single, closed, and never branches. So at every dot, either no lines meet or exactly two do. Never one, never three. This sounds small and it decides the whole grid. A line that arrives at a dot must leave by exactly one other edge, so the moment two of a dot's edges are crossed out, an arriving line has only one way onward: draw it. And if three edges at a dot are dead, the fourth is dead too, because a line cannot simply stop there. Count line-ends at dots constantly. It is the engine every other trick runs on.
One more consequence: you may never close a small loop early. The finished answer is one loop satisfying every clue, so if joining two line-ends would seal a ring while unsatisfied numbers remain outside it, that join is illegal and you can cross it out. Late in a puzzle this rule alone places the last several edges.
Patterns worth memorising
- A 3 in a corner: the two outer edges of that corner cell are always in the loop.
- A 1 in a corner: its two outer edges are always dead.
- Two 3s side by side: the edge between them is used, plus the two outer edges facing away from each other.
- Two 3s touching diagonally: the outer corner edges of both cells are drawn.
- A 0 next to a 3: three of the 3's sides sit away from the 0, and lines spill out along the diagonals.
- A line arriving at one corner of a 3: the two sides of the 3 away from that corner are both in the loop.
Here is why the corner 3 works, using nothing but the dot rule. The border corner dot touches only two edges, the cell's two outer ones, so a line at that dot must use both or neither. A 3 can only skip one of its four sides, so it cannot skip both outer edges; and it cannot use just one of them, since that line would dead-end at the corner. Both outer edges are in. Every pattern above falls to the same kind of short argument, which is why proving one properly teaches you the rest.
A sensible order of attack
Open every puzzle the same way. First pass: cross out all four edges of every 0, then stamp in the corner patterns and adjacent-3 patterns wherever they appear. Second pass: walk the border, because border dots have fewer edges and force lines early. After that, follow the loop ends. Each time a line grows, check the dots and numbers it just touched, since new forced moves cluster around fresh ink. When nothing moves, hunt for a cell one edge short of its number with only one candidate edge left. And if you are truly stuck, re-scan your crosses: the missing deduction is nearly always a dot that already has two dead edges and a line waiting to be steered.
From Nikoli, 1989
Slitherlink first appeared in 1989 in Puzzle Communication Nikoli, the Japanese magazine that later drove the worldwide Sudoku boom. Nikoli credits reader submissions for many of its formats, and this one grew popular enough to fill dozens of dedicated books. English publishers have printed it as Fences, Loop the Loop and Takegaki. Unlike a number-placement puzzle, the answer is a single unbroken loop threading the grid, and a properly set grid has exactly one. That guarantee is what makes the cross-out habit safe: every edge you kill by logic stays dead, and the loop that remains at the end was there from the start, waiting to be uncovered.
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Slitherlink: frequently asked questions
What does each number mean?
It’s how many of that cell’s four edges are part of the loop: a 0 means none, a 3 means three of the four.
Can the loop branch or cross?
No. There’s exactly one closed loop with no crossings, splits, or loose ends.
What is the most useful habit for solving Slitherlink?
Mark the edges the loop cannot use, not just the ones it uses. A crossed-out edge constrains the numbers around it just as much as a drawn line, and ignoring them makes the puzzle far harder than it needs to be.
What does a 0 clue mean?
A 0 means the loop uses none of that cell's four sides, so you can rule out all four edges straight away. Those marks then help pin down the cells next to it.
What does a number in a cell tell me?
It tells you exactly how many of that cell's four sides are part of the loop. A 3 uses three of its four edges, a 1 uses one, and a cell with no number can use any amount.
How does the single-loop rule help?
The loop never branches or dead-ends, so at every dot the line either passes through, turns, or is absent: each dot has zero or exactly two line-ends. That fact resolves a huge number of cells once you rely on it.
Who created Slitherlink?
It first appeared in 1989 in Puzzle Communication Nikoli, the Japanese magazine behind the Sudoku boom, which credits its reader community for many of its puzzle formats. In English it also goes by names such as Fences and Loop the Loop.
Why can I not close a small loop early?
Because the finished solution is exactly one closed loop that satisfies every clue. The moment any loop closes, the puzzle is over, so sealing a small ring while numbers elsewhere remain unsatisfied is automatically wrong. You can cross out any join that would do it.
Is Slitherlink free to play?
Yes. Slitherlink is completely free on Inkplay, with no download, no sign-up, and no in-game purchases. Open the page and play instantly on desktop or mobile.
Can I play Slitherlink unblocked?
Yes. Slitherlink runs entirely in your browser with no download, install, or plugins, so it loads instantly and keeps working even on school or work networks where bigger game sites get blocked.
More games to play
In the mood for more? Try 2048, Memory Match, Lights Out, Minesweeper, or browse all puzzle games and the full games list on Inkplay. Slitherlink also appears in Logic Puzzles.