The present invention proposes a cap having a collar portion with three or more contact regions arranged to contact the end of a fastener around which the collar portion is provided, the collar portion having a lobed shape in cross-section such that an outwardly projecting lobe extends between each neighboring pair of contact regions. An advantage of this lobed arrangement is that the cap can tolerate relatively large manufacturing tolerances at the fastener end. For example, the cap may be for fitting over a nut or fastener head, with the contact regions of the cap being arranged to contact corresponding contact regions on a cylindrical collar base portion of the nut or fastener head. The diameter of this cylindrical collar portion may not be well controlled, so that the manufacturing tolerance is high. The lobed shape of the cap base ensures that the contact regions will always provide a good frictional engagement with the nut, whether the cylindrical collar is undersized, oversized, or at its nominal diameter.