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Anson Chan
Anson Chan

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How to Read Empty Cells in Slitherlink Puzzles

#tips #puzzles #strategy

The number 0 in Slitherlink is the most underrated clue on the board. New players see it as "nothing happens here" and move on. Experienced solvers see it as the fastest deduction in the game — and a catalyst for solving everything around it.

What 0 Means

Zero lines surround this cell. All four edges are empty. Mark them all with × immediately.

·   ·   ·
  × 0 ×  
· × · × ·
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No thinking required. No conditions to check. Just eliminate all four edges and move on. This takes one second and removes four edges from consideration permanently.

Why 0 Is So Powerful

The power isn't in the 0 itself — it's in what it does to its neighbors.

0 Next to 3

When a 0 sits next to a 3, their shared edge is eliminated by the 0. The 3 originally had 4 edges and needed 3 of them. Now it has only 3 edges left — and needs all 3. Every remaining edge of the 3 must be a line:

·   ·───·   ·
× 0 × 3 │    
· × ·───·   ·
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The 0 eliminates 4 edges. The shared edge elimination forces all 3 remaining edges of the 3. That's 7 edges resolved from just two clues.

0 Next to 2

A 0 next to a 2 eliminates their shared edge, leaving the 2 with 3 remaining edges and needing 2 lines. This doesn't fully resolve the 2, but it reduces its possibilities from 6 combinations to just 3.

0 Next to 1

A 0 next to a 1 eliminates the shared edge. The 1 now has 3 edges and needs exactly 1 line. Combined with vertex rules at the shared vertices, this often cascades into further deductions.

0 in a Corner

A corner 0 is even more constrained because a corner cell only has 2 outer edges (touching the grid boundary) plus 2 inner edges. All four are eliminated:

· × ·   ·
× 0 ×    
· × ·   ·
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The corner vertex and the two edge vertices all get × marks, which propagates vertex rule constraints to adjacent cells.

0 on an Edge

An edge 0 (non-corner, touching one grid boundary) has 3 inner edges plus 1 outer edge. All eliminated. The outer edge × mark is particularly valuable because edge vertices only have 3 edges total — eliminating one leaves just 2, making vertex rule deductions very likely.

Multiple Adjacent 0s

When two 0s are adjacent, their combined elimination is massive. A horizontal pair of 0s eliminates 7 unique edges (they share one):

·   ·   ·   ·
  × 0 × 0 ×  
· × · × · × ·
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A 2×2 block of four 0s eliminates 12 edges. In a small grid, this can resolve a third of the puzzle instantly.

The 0 Scanning Strategy

At the start of every puzzle:

  1. Find all 0s — scan the entire grid
  2. Mark all their edges × — this is instant, no deduction needed
  3. Check every neighbor of every 0 — look for 3s (fully resolved), 2s (constrained), 1s (constrained)
  4. Apply vertex rules at all affected vertices — the × marks from 0s will cascade

This 30-second process often solves 20-30% of an easy puzzle before you do any real thinking.

0 as a Connectivity Clue

Beyond edge elimination, 0s create "walls" in the puzzle. The loop cannot pass through any edge of a 0-cell, which means the loop must flow around it. In tight spaces, a cluster of 0s can force the loop into a narrow corridor, making the path nearly obvious.

Practice 0-heavy puzzles: slitherlinks.com — start with 5×5 Level 1, where 0s and 3s dominate the grid.

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