How Does Slitherlink Work? Why You Draw a Loop, Not Fill in Numbers

The first time you see Slitherlink, your brain plays a trick on you. There is a grid, there are numbers, so surely you fill in the empty squares like a Sudoku? Not at all, and that misunderstanding is exactly why some people bounce off this brilliant puzzle before they have given it a chance. Slitherlink works in a way that is genuinely different from almost every other number-grid puzzle: you never write a single digit. Instead you draw a loop, and the numbers are only there to guide you. Here is how Slitherlink actually works, and why that one difference makes it feel so fresh. Want to see it in action? Play a Slitherlink puzzle as you read.

The big idea: you draw, you don't fill

Here is the heart of it. In Sudoku, Suguru, KenKen and most number puzzles, you write numbers into cells. In Slitherlink, you do something completely different: you draw a single continuous loop along the lines of the grid, the edges that run between the dots at the corners of the cells.

So you are not working with the cells at all. You are working with the edges around them. Every edge is either part of your loop or not, and your whole job is to decide which edges the loop uses so that it forms one neat closed circuit. The numbers printed in some cells are clues, not answers you place.

What the numbers actually mean

This is the piece that unlocks the puzzle. A number in a cell tells you exactly how many of that cell's four sides are part of the loop. Nothing more.

• A 0 means the loop uses none of that cell's four edges. The loop avoids it entirely.
• A 1 means one of the four edges is part of the loop.
• A 2 means two of them are.
• A 3 means three of the four edges are used, so the loop wraps tightly around the cell.

Cells with no number are unconstrained: the loop may use any number of their edges, including none. So the numbers are not values to enter, they are counts to satisfy. Once that clicks, the whole puzzle reframes itself.

The single-loop rule

The other half of Slitherlink is the shape your drawing must take. When you are finished, the edges you have drawn must form one single closed loop, and only one. That means:

• The loop is continuous: you can trace the whole thing without lifting your pen.
• It never crosses itself and never branches. At any dot, the loop passes straight through using exactly two edges, or it does not touch that dot at all.
• There are no loose ends and no separate little loops. Everything joins into one circuit.

That single-loop requirement is what makes Slitherlink so satisfying, and it is the constraint that lets you solve it by logic. (For the actual solving moves and patterns, our tips and techniques page is the place to go.)

Why it's nothing like Sudoku

People reach for the Sudoku comparison because both have a numbered grid, but the two puzzles could hardly be more different in how they play:

	Sudoku	Slitherlink
What you do	Write digits into cells	Draw a loop along edges
The numbers are	Answers you place	Clues that count edges
You interact with	Cells	The edges between cells
The goal	Fill every cell correctly	Form one closed loop

In Sudoku the numbers are the whole point, the thing you produce. In Slitherlink the numbers never change; they just whisper how many edges of each cell belong to the loop. If you have only ever met grid puzzles where you fill in numbers, Slitherlink is a genuinely new experience, which is a big part of its charm. (It also goes by several other names, as our piece on Slitherlink, Loop the Loop and Fences explains.)

So how do you start one?

Once the mechanic makes sense, starting is easy. The friendliest opening move is to look at any 0 clues, because you know none of their edges are used, so you can rule them out straight away. From there, the numbers and the single-loop rule steadily force which edges must be drawn and which must be left out, until the loop completes itself. There is always a logical path, with no guessing required. The full set of opening moves and patterns lives on our tips page.

The short version to remember: Slitherlink is a loop puzzle wearing a number puzzle's clothes. You draw, you do not fill, and the numbers are guides, not goals. Once that idea lands, a puzzle that looked baffling suddenly makes perfect sense. Play a Slitherlink puzzle now, or learn the full rules first.

Frequently asked questions

How does Slitherlink work?

In Slitherlink you draw a single closed loop along the edges of a grid. The numbers printed in some cells tell you exactly how many of that cell's four sides the loop must use, from 0 (no edges) up to 3 (three edges). You never write numbers into cells; you decide which edges the loop uses so it forms one continuous circuit with no branches or crossings.

What do the numbers mean in Slitherlink?

Each number is a clue that counts edges: it states how many of that cell's four sides are part of the loop. A 0 means none of its edges are used, a 3 means three of the four are. Cells with no number can have any number of their edges used. The numbers are not values you enter, they are constraints to satisfy.

Is Slitherlink like Sudoku?

No. Although both have numbered grids, they play completely differently. In Sudoku you write digits into cells, while in Slitherlink you draw a loop along the edges between cells and the numbers are only clues. Slitherlink is a loop puzzle, not a number-placement puzzle.

Do you fill in numbers in Slitherlink?

No. You never fill in numbers in Slitherlink. The numbers are given as clues, and your task is to draw a single closed loop along the grid's edges so that each numbered cell has exactly that many of its sides used by the loop.