A container, such as a flask or a flexible tube, provided with a cap, the container including a side wall and a plastic head with a neck and a shoulder linking the neck to the side wall. The cap is provided with a circular cylindrical skirt on its inner surface with one or more first screwing means. The wall of the neck is substantially shaped like a prism where the cross-section is globally a polygon having a symmetry of order n about an axis A substantially parallel to the generatrix of the prism, n being an integer strictly greater than 1, and the neck is provided on its outer surface with n second screwing means, uniformly distributed about the axis A. The assembly of n second screwing means of the neck co-operates with the first screwing means of the cap.