Corner Clue

Easy Used by Smart Hint

A grid corner's two outer edges always match — both lines or both blank, since the corner dot only touches those two edges. A clue of 1 or less rules both out; a clue of 3 or more forces both in.

Edge ruled out
Cells that establish the pattern

How it works

This cell sits in a grid corner — its corner dot touches only these two edges (top and left), nowhere else to go.
A loop can never dead-end, so that dot needs exactly 0 or 2 lines — meaning top and left always match: both lines, or both blank.
With a clue of 1, both being lines would already total 2 — too many. So both must be blank. (A clue of 3 or more works the same way in reverse: both would have to be lines.)