The white pieces are sections of crown molding. This is the molding between a wall and the ceiling, to make a room look beautiful while hiding cracks. Since the corners are not totally perpendicular, the molding sometimes does not align properly. So what I did here was make the corners first -- cutting the right pieces using 45 degree angle miter cuts -- gluing them together (held together by black tape), with small pieces to help align the sections. I may strengthen the corners (both outside and inside corners) with small screws out of sight. It is part of a kitchen renovation project.