The Short Loop Trap: Why Your Solution Breaks
A valid Slitherlink solution is ONE loop. Learn to detect and prevent accidental short loops that invalidate your work.
The One Loop to Rule Them All
The cardinal rule of Slitherlink is: One Single Continuous Loop.
A common beginner mistake is to successfully satisfy all the numbers but end up with two or three separate, closed loops. This is an invalid solution. Worse, making a small "short loop" early in the game can render the puzzle unsolvable later.
What is a Short Loop?
A "short loop" is any closed loop that does not encompass the entire solution. It's a small circle of lines that closes off on itself, leaving other numbered cells or lines stranded outside of it.
✗ SHORT LOOP (invalid) ✓ SINGLE LOOP (valid)
·═══·═══· · · ·═══·═══·═══·═══·
║ 2 ║ 2 ║ 2 2 ║ 2 ║ 2 2 2 ║
·═══·═══· · · ·═══· ·═══· ·
║ 2 ║ 2 ║
Two loops! Numbers ·═══·═══·
outside are stranded. One continuous loop ✓
How to Detect the Trap
As you solve, you will often have long "snakes" of lines growing across the board. The danger comes when you are about to connect two ends of a snake.
The Check: Before you draw a line that connects two endpoints, ask yourself: "If I connect these, does it close the loop? And if it closes the loop, is the rest of the puzzle finished?"
- If the answer is "Yes, it closes the loop" but "No, the puzzle isn't finished," then that connecting line is forbidden. Mark it with an X immediately.
Prevention Strategy: The "Snake" Mindset
Think of your solution as one or two growing snakes. You want to keep the head and tail separate until the very end.
If you have a U-shaped segment and you see a potential move that would cap off the U into an O, check if that O covers everything. If it's just a tiny O in the corner, you know you can't close it. You must extend the snake outward instead.
✗ Premature closure ✓ Keep extending
·═══·═══· · · ·═══·═══· · ·
║ ║ ║
· · · · · · · · · ·
║ ║ ║
·═══·═══· × · × · · ·═══·═══· ·
(closes too early!) ║ ║
Can't reach rest of grid! ·═══·═══·═══·═══·
Critical Endgame Checks
This logic is most critical in the middle-to-endgame. You might have two large separate segments. Connecting them is good—it merges two snakes into one bigger snake. But connecting the head of Snake A to the tail of Snake A is fatal unless Snake A is the only thing left.
Practical Tip
If you are unsure whether a move creates a short loop, trace the path with your finger (or mouse). Follow the line from one end of your proposed connection. If you arrive back at the other end without visiting the rest of the unsolved board, you've found a short loop. Block that connection.
Trace test: follow the path ·═══·═══· Start at A, follow arrows: ║ ║ A → B → C → D → back to A A · B without visiting E, F, G... ║ ║ D═══·═══C E · F ← These are stranded! Result: This is a short loop. Mark × to block it.