Abstract: Corner strips are provided for positioning between perpendicularly-disposed wallboard sheets to effect a curvilinear transition between the sheets. In one embodiment, a longitudinally-extending and transversely concave or inwardly curved rigid member is adapted for positioning between the sheets with the sheets perpendicularly-disposed in mutually-abutting relation. The concave member is held in the corner by an arrow-like member engaging putty-like material placed in the corner. In another embodiment, a longitudinally-extending and transversely convex or outwardly-curved rigid member is adapted for positioning between the sheets with the sheets perpendicularly-disposed in spaced-apart relation. The convex member is held to the sheets by a pair of open channels engaging the edges of the sheets. In both embodiments, wallboard tape, bonded to the concave and convex members, overlaps the wallboard sheets and effects a smooth transition between the sheets and the corner strip members.