Dot Rule

Easy Used by Smart Hint

Every dot needs exactly zero or two lines touching it, never one. A dot with two lines already has its rest ruled out; a dot with only one edge left has that edge forced by how many lines it already has.

Edge on the loop
Edge ruled out
Cells that establish the pattern

How it works

This dot already has two lines meeting at it.
A loop can only pass through any dot once — degree 0 or 2, never more.
Its other two edges are impossible now — mark them with an X.