Hard tier Technique 20 of 31 Used by Smart Hint

Three 3s in a Row

3-3-3 side by side: all four vertical edges crossing the run are lines.

How it works

1Read it as two overlapping adjacent pairs rather than as one new pattern.
2The left pair is two adjacent 3s, so their outer edges are lines: the far-left edge, and the edge between the middle and right cells. The right pair gives the other two the same way.
3Between them that is every vertical edge crossing the run — four lines from three clues, and the whole run is fenced.
4Each 3 now has its left and right drawn, two of the three it needs, so each has exactly one of its top and bottom. The run is fenced, but which way it bulges is still open. A vertical 3-3-3 works identically, with the four horizontal edges.

Edge the technique adds
Cells that establish the pattern

Next in the ladder Sector Chain →

A sector is the pair of a cell's sides meeting at one of its corners, and the loop uses 0, 1, or 2 of them. A clue's opposite corners sum to it, so what travels is whether that count is odd or even — a 2 keeps it, a 1 or 3 flips it, and dots pass it on diagonally. A chain seeded at a border forces edges far from where it started.