The rules of slitherlink
Slitherlink is a logic puzzle by Nikoli (the Sudoku company). Draw a single closed loop along the grid lines so that every numbered cell has exactly that many of its four sides used by the loop. Cells without a number impose no rule at all. Every puzzle has exactly one solution.
The five rules
- Draw a single closed loop along the grid lines — it never crosses or branches.
- Every numbered cell must have exactly that many of its four sides used by the loop.
- Cells without a number impose no rule at all.
- The loop may touch itself at corners, but it must be one continuous, unbroken ring.
- There is exactly one solution, reachable by logic.
How to read the grid
Each number sits inside a cell and says exactly how many of that cell's four sides your loop must use. A 0 means none of its sides are drawn on; a 1 means exactly one; a 2 means two; a 3 means three. Blank cells are free — the loop may use any number of their sides, including none.
The loop
Your answer is one continuous ring drawn along the grid lines, like a snake with its tail in its mouth. It can turn corners, but it can never cross itself, branch into two paths, or stop partway. A common beginner mistake is to close a small loop early — the puzzle only ever has one loop, so any closed ring that isn't the full solution is a wrong turn.
One more thing: crossing out
Just as important as drawing lines is marking the lines you know are not used. On screen, click a grid line to draw it, click again to cross it out as a note, and click a third time to clear it. On paper, an × on a line marks it as definitely not part of the loop — most solving techniques produce these "off" lines just as often as they produce drawn ones.