How to play
Slitherlink

Leave a way through.

One line leads to the next. Follow the small certainties until the whole loop comes home.

Count the sides. Just the sides.

A Slitherlink clue is a promise: exactly this many of the cell’s four sides belong to the loop.

Watch three sides grow around this 3. The fourth stays unselected. This is an example of the rule; we have not proved any puzzle moves yet.

Three sides, exactly. Not three dots or three cells.

What is this 3 counting?

Choose an answer. Follow the reason.

Read the answer

Exactly. The top, right and bottom make three sides. Corners and area do not count.

Ochre dashes are an example. Solid teal lines will mean proved moves.

Begin with nothing.

Now let the neighbours tell us which sides to use. Our example clears, and the same 3 becomes part of a real puzzle.

The 0 next door needs no lines. Cross all four of its sides. A blank cell is different: it has no count to meet.

The 0 needs no lines. Each of its four sides is excluded.

A cross means excluded. A faint dashed edge is still undecided.

One side. Two clues.

The 0’s right side is also the 3’s left side. That cross belongs to both cells.

The 3 still needs three lines. Only three sides are available. Draw the top, right and bottom. Nothing is guessed.

The 3 has one side excluded. All three remaining sides must be lines.

Shared edges count for both neighbouring cells when drawn, and are excluded for both when crossed.

Leave the dot a way out.

Look at the highlighted dot above the shared cross. One line arrives. Two other directions are crossed. Only the upward edge can be its second line.

That new line also gives the 2 above its second side. Its other sides are now excluded. At the next unused dot, a lone possible edge is excluded too: it cannot form a dead end.

At this dot: one line, two crosses. The only open edge must continue upward.

The upper-right 2 now has two lines. Its top and right sides are excluded.

This unused dot has two crossed directions. Its last edge cannot be a lone line.

Two lines already meet at this dot beside the 3. A third would branch.

Could a third line leave the highlighted dot?

Choose an answer. Follow the reason.

Read the answer

Right. The path arrives and leaves: two lines. A third would branch; a fourth would cross. Neither is allowed.

In a finished puzzle every used dot has two lines. Unused dots have zero. No ends, branches or crossings.

The path carries its own next step.

Keep following the open end. The top dot has one line and one cross, so it must continue left.

The next 2 is satisfied. Excluding its remaining side leaves a way left again, then down at the corner. A path can go straight or turn.

One line reaches the top dot; the right edge is crossed. Continue left.

This 2 has its top and right lines. Its remaining left side is excluded.

One line arrives here and the downward edge is crossed. Continue left.

At the corner there are only two edges. One is a line, so turn down.

The blank corner is allowed to have lines. Blank does not mean zero.

Small certainties travel.

At the dot inside the upper-left corner, three directions are excluded. The fourth must be excluded too. That gives the path beside it a way down.

Alternate the same two checks: has a clue got all its lines, and can this dot have zero or two? Each small answer makes the next one possible.

Three directions here are crossed. The last cannot carry a line by itself.

The left-edge dot has a line above and a cross to the right. Continue down.

This 1 already has its left line. Its bottom side must be excluded.

One line above, one cross right: continue down the left edge.

This unused dot has three crossed edges. Exclude its last edge too.

The lower-left 2 has one line and two crosses. Its bottom must be a line.

Watch the highlighted clue or dot. Earlier proved lines and crosses stay in place.

Now you can come home.

The last 2 has its top and left sides excluded. It needs both remaining sides: right, then bottom.

The last connection joins our two open ends. Every clue matches, every used dot has two lines, and all lines are one connected loop. Now closure is exactly right.

The last 2 has only two available sides. Draw both to join the open ends.

The whole board: every clue matches. One closed loop, with room to spare.

A loop need not visit every dot or enclose every clue. The upper-right 2 is outside this one; the 0 is inside.

One loop means one.

Try adding this deliberately wrong little loop in the blank corner. Every numbered clue still matches. But there are now two separate loops, so the puzzle fails.

A closed component cannot grow without branching. Closing early is wrong when it abandons required lines or unfinished clues elsewhere. The closure we just proved completes the puzzle, so it is allowed.

Incorrect example: two separate closed loops fail.

Keep the single proved loop. Unused edges may remain undecided.

The rust loop is an incorrect example, never a proved move. Let it go; keep the one you earned.

Find your own way through

A fresh set of dots.
Your first loop.

A different, original 5 × 5. Begin with a 0, then let a clue or a dot give you one sure move at a time. No timer. No prefilled lines.

Choose a cell, then choose its side.

Start with a 0. Cross its four sides, then look at the dots nearby.

Tap a cell. Mark a side.

The large side buttons change the selected cell’s top, right, bottom or left edge. Shared sides change for both neighbours.

Practice stays here while this page is open. Your ordinary Slitherlink saves are separate.

Controls & notes

Select a cell with a tap or the arrow keys. Tab reaches Top, Right, Bottom and Left. Enter or Space marks that side. Choose Line, Cross or Erase first; repeating Line or Cross clears it.

L, C and E switch tools. Ctrl/Cmd+Z undoes. No dragging is needed, so you can scroll normally. The selected side buttons announce their current marks.

Crosses are useful notes. The puzzle is complete when the lines form one loop and every clue matches, even if unused edges are still undecided.

Continue to the game

Your lines will appear here as you draw them.

Keep a way open.

Count the sides. Continue at the dots. Bring every line into one loop. Your next puzzle is ready whenever you are.

Play Slitherlink

Free to play. No account needed.

The quick rules.

Draw one closed loop on edges between adjacent dots.
A clue is the exact number of its four sides used by the loop. It does not count corners, dots or area.
A blank cell has no count. A 0 excludes all four sides.
Every used dot has exactly two incident lines; unused dots have zero. Turn or go straight, without branches, crossings or ends.
Match every clue and connect all lines in one loop. Separate loops fail.

Crosses mark excluded edges. Faint dashes are undecided. Crosses on unused edges are optional for completion.

Follow the guided proof Try the starter

A little more clarity.

Does the loop have to enclose every number?

No. A number can be inside or outside the loop. Only its surrounding side count matters. The worked puzzle’s upper-right 2 is outside its loop.

Must every dot be used?

No. An unused dot has zero lines. If the loop uses a dot, it must arrive and leave on exactly two edges.

Can I close a loop early?

A closed component cannot grow without branching. If required lines or clues are left unfinished elsewhere, that closure is wrong. A closure that genuinely completes a valid puzzle is allowed.

What if my choices stop working?

Undo or erase a recent mark. Explain a move checks your current lines and crosses. It either explains a forced edge, reports a contradiction, or says its bounded check found no simple move. Apply makes an explained move separately.