A structure comprising a plurality of plies of fiber-reinforced composite material, each ply having a ply orientation defined by a principal stiffness direction of the ply. The structure has a first region and a second region, each region comprising N1 primary plies, N4 cross plies with a ply orientation substantially perpendicular to the primary plies, N2 positive-angled bias plies with a ply orientation at a positive acute angle to the primary plies, and N3 negative-angled bias plies with a ply orientation substantially perpendicular to the positive-angled bias plies. The ply orientation of the primary plies in each region is more closely aligned with a primary axis of the structure than the cross plies or the bias plies. N1=N4 in the first region but not in the second N region, and N2=N3 in the second region but not in the first region. In each of the two regions, one ply set is balanced and the other ply set is un-balanced. This provides a compromise between the advantages of using un-balanced ply sets, for instance to provide bending torsion coupling, and the disadvantage that un-balanced ply sets can introduce thermally induced distortion during manufacture.