Abstract: The invention concerns an apparatus comprising first and second planar elements, e.g. of cardboard, which may be assembled together to form a structure, e.g. a cross brace of a bottle crate. Each of these elements has a slit which extends to an edge of the respective element and into which the other element may be introduced. The slits of the two elements have linear portions which are aligned in a predetermined direction when the elements are assembled. The slit of the first element is formed to provide a flap which extends transversely with respect to the predetermined direction and which is pivotable about a folding line which extends obliquely with respect to the predetermined direction. The second element has an aperture in which the flap is engaged with the elements are assembled.