Tiles that get touched every time
Intermediate technique
How can a region rule out tiles outside itself?
If every possible arrangement of a region's stars puts a star next to the same outside tile, that tile can never hold a star, even though you don't yet know which arrangement is right.
This is the step that opens most medium boards. It works best on narrow regions: a region that is a line of three tiles must put a star on at least one end and often the middle, so the tiles alongside its centre are touched whichever way it goes.