Corner and Edge Patterns Every Player Should Know

Why Borders Matter

In Slitherlink, the edges of the grid are your best friends. Because the loop cannot go "off the board," clues located on the perimeter are significantly more constrained than those in the middle. Mastering these boundary patterns is the fastest way to improve your speed.

Corner Configurations

The four corners of the grid are the most restrictive spots. Let's break down exactly what happens when a number lands in a corner:

• "3" in a Corner: This is the most powerful opening move. A 3 in a corner forces lines on both outer edges. Why? If either outer edge were empty (X), the loop would have to go "in" and "out" through the remaining two internal edges, which would form a small, illegal closed loop or leave a loose end. So, always draw the two corner lines immediately.

·───·   ·
│ 3      
·   ·   ·

• "0" in a Corner: A 0 means no lines touch it. In a corner, this instantly gives you two Xs on the outer edges. It effectively "bites" the corner off the grid.
• "1" in a Corner: This is tricky. A 1 in a corner has only two valid states: either both internal edges are lines (and the corner itself is empty), or one outer edge and one internal edge connect. However, you can definitely say that the two outer edges cannot BOTH be lines.

· × ·   ·
× 1      
·   ·   ·

• "2" in a Corner: A 2 in a corner implies that the loop either goes "through" the corner (two outer lines) or "bypasses" it (two inner lines). If you find a line entering a corner 2 from the outside, it must continue along the outside.

·───·   ·
│ 2      
·   ·   ·

Edge Patterns

Moving away from the corners, let's look at the sides of the grid.

• Adjacent "3"s along an Edge: If you see two 3s side-by-side along the grid border, you can fill in three lines: the shared edge between them, and the two outer edges. This also forces the loop to turn inward at both ends.

·───·───·───·
    │ 3 │        
·   ·   ·   ·

• "3" next to "0" on an Edge: A 0 blocks all its neighbors. If a 3 is next to it along the wall, the 3 only has 3 available sides remaining. Therefore, all 3 available sides must be lines. This is an automatic fill.
• Edge Cells with "2": A 2 on the edge often acts as a straight pipe. If a line hits a side "2" perpendicular to the wall, it cannot turn along the wall (that would require 2 lines for one connection). It often must go straight through or bounce off.

Chaining Deductions

The real magic happens when you chain these. A "3" in the corner gives you lines. Those lines might travel along the edge and hit a "2", forcing it to extend further. Always scan the entire perimeter of the grid first. You can often build a "frame" for your solution before tackling the chaotic middle.