Slitherlink Rules, Loop Strategy, and How to Play Online
Slitherlink is a logic puzzle about drawing a single fence. The board is a grid of dots,
and some of the squares between them hold a number from 0 to 3. Your job is to connect the
dots into one closed loop so that every number tells exactly how many of its four sides the
loop uses. A 0 is never touched, a 3 is wrapped on three sides, and an empty square can be
anything.
Every board has exactly one solution, checked by a solver, so you never need to guess: the
numbers always contain enough information to find the loop. Play Slitherlink online for
free on 5x5, 7x7 and 10x10 boards in easy, medium and hard puzzles, with a new daily
puzzle, hints, undo and redo, and saved progress. This page explains the rules, the
controls, and the patterns that solve most boards.
How to Play Slitherlink
Choose a board size and a difficulty. Tap the space between two neighboring dots to draw a
line there. Tap the same edge again to cross it out, a note that says "the loop doesn't go
here", and once more to clear it. Drag along the edges to draw or cross several in one
gesture.
- One loop: all the lines must join into a single closed loop with no
loose ends.
- No crossings or branches: exactly two lines meet at a dot on the loop,
and none at the dots it skips. The loop never crosses or touches itself.
- Numbers count sides: a number is how many sides of its square are part
of the loop. Squares without a number have no limit.
Numbers fade once they have all their lines and turn red when they have too many, and a
red dot shows where three or four lines meet. The puzzle is solved as soon as your lines
form the loop; crosses are only notes and don't need to be complete.
On a keyboard, the arrow keys or WASD move a cursor between the dots. Hold Shift with a
direction to draw a line (or erase it) and move along it, and hold Ctrl (Cmd on a Mac) with
an arrow to cross the edge out. Press H for a hint, Shift+Backspace to clear all crosses,
and Ctrl+Z or Ctrl+Shift+Z to undo and redo.
On big boards, pinch or use the zoom buttons to get closer, and drag outside the board to
move it around. The center button, or C, brings the whole board back into view.
Slitherlink Strategy for Beginners
The fastest way into a puzzle is to cross out everything you can rule out, then look for
dots that only have one way left to go.
- Start with the zeros: cross out all four sides of every 0. Those crosses
often force lines on the squares next to them.
- Two lines per dot: a dot with one line and only one open edge must
continue through it. A dot with two lines crosses out its other edges, and a dot with
three crosses can't be on the loop at all.
- A 3 in a corner: the two sides along the board's edge are always part of
the loop, because the loop can't turn around the corner dot from outside the board.
- A 1 in a corner: the two outer sides are always crossed out. The loop
can't touch the corner dot with just one of them.
- Adjacent 3s: two 3s side by side always share the edge between them and
have lines on their outer sides, parallel to it.
- Diagonal 3s: two 3s touching at a corner always have lines on their far
corners, the two sides of each that point away from the other.
Advanced Slitherlink Techniques
Medium and hard puzzles have fewer numbers, so the local patterns run out sooner. These
ideas carry you through.
- Don't close the loop early: if one edge would join the two ends of the
same line into a small loop while other lines still exist, cross it out.
- Inside and outside: the loop splits the board into an inside and an
outside. Two squares on either side of a line are on opposite sides; two squares on
either side of a cross are on the same side. Squares beyond the border are always
outside. Following these chains tells you whether a far-away edge must be a line.
- Counting with sides: a 3 is inside or outside while three of its four
neighbors are on the other side, and a 1 shares a side with three of its neighbors. When
two neighbors are known to be on the same side as each other, their edges to the number
match: both lines or both crosses.
- Try an edge: on hard boards, imagine an edge as a line and follow the
forced moves. If they lead to a contradiction, such as a number with too many lines or a
dot with three, the edge must be crossed out.
Difficulty Levels
Easy puzzles fall to the counting rules alone: numbers, two lines per dot and no early
loops. Medium puzzles also need inside and outside reasoning to pass information across
the board. Hard puzzles have the fewest numbers and need you to test an edge and see where
it leads. Bigger boards add length rather than new rules, so 10x10 hard is the longest
challenge.
Common Mistakes
- Drawing a line before checking both of its dots: a dot can never have three lines.
- Forgetting that empty squares can have any number of lines, including 0 or 3.
- Closing a small loop that leaves numbers unsatisfied elsewhere on the board.
- Skipping crosses. Marking the edges that are ruled out is what reveals the forced ones.
History of Slitherlink
Slitherlink first appeared in 1989 in the Japanese puzzle magazine Puzzle Communication
Nikoli, the same publisher that popularized Sudoku. Its name plays on "slither" and
"link", for the loop that winds around the board. It has also been published as Fences,
Takegaki and Loop the Loop, and it is now a classic of the pencil-and-paper logic puzzle
family, alongside Hashi, Hitori and Nurikabe.
Slitherlink FAQ
What are the rules of Slitherlink?
Connect the dots into one closed loop that never crosses or branches, so that every number
equals how many sides of its square the loop uses.
What does a 0 mean in Slitherlink?
None of the square's four sides are part of the loop, so you can cross them all out.
Is there always only one solution?
Yes. Every board on Lofi and Games is checked by a solver and has exactly one loop that
follows the numbers.
Do I have to cross out every edge?
No. Crosses are notes that help you think. The puzzle is solved as soon as your lines make
the loop.
Which board should I start with?
Start with easy 5x5 to learn the zero, corner and 3 patterns, then move on to 7x7 and the
medium puzzles.
Ready to draw? Cross out the zeros, trace the corners, and follow the dots that only have
one way to go. One forced edge leads to the next, until a single loop closes around the
board.
Sound Effects Credits
The sound effects used on the game come from multiple parties. The credits and
respective licenses are listed below:
Disclaimer
This game is a property of Lofi and Games. All code and assets are protected and must
not be redistributed or used without prior permission.