By "exterior corner" I mean the corners of the exterior walls of the
house--these are, of course, interior (cove) corners of the drywall.
Anyway, lumber shrinks from moisture loss just twice as much radially as
it does tangentially, but of course almost not at all lengthwise (it
does expand and contract lengthwise to a measurable degree with
temperature changes). A 2x6 should shrink .1", 5.5" to 5.4" going from
19% to 12% moisture content, and I suppose that is what is causing the
trouble. I intend to put acrylic caulk in the crack when I paint.