A container, comprising: a thread arranged around an opening of the container for engaging a corresponding thread of a closure; more than four retention members arranged around the opening, each retention member having an end-face for abutting a member on an interior surface of the closure to impede unthreading of the closure; wherein the retention members are arranged such that a tangent to each end-face of the retention members intersects a mould splitting plane (160, 350) bisecting the container at an obtuse angle; and characterised in that the retention members are arranged into two first retention members (141, 410) and two groups (142, 420) of retention members the first retention members (141, 410) each having an end-face substantially parallel to the plane bisecting the container, and a first one of the first retention members (141, 410) and one of the groups(142, 420) is arranged on one side of the plane (160, 350), each in a respective quadrant of the container, and the other one of the first retention members (141, 410) and the other one of the groups (142, 420) is arranged on the other side of the plane (160, 350), each in a respective quadrant of the container; wherein teeth of each of the groups (142, 420) have a first inter-tooth spacing of substantially one tooth-length, and an inter-tooth spacing between an end-face of the first one of the first retention members (141, 410) and a first retention member of the group (142, 420) on the one side of the plane is larger than the first inter-tooth spacing, and wherein an inter-tooth spacing between the end-face of the first one of the first retention members (141, 410) and a first retention member of the group (142, 420) on the other side of the plane is also larger than the first inter-tooth spacing. a thread arranged around an opening of the container for engaging a corresponding thread of a closure; more than four retention members arranged around the opening, each retention member having an end-face for abutting a member on an interior surface of the closure to impede unthreading of the closure; wherein the retention members are arranged such that a tangent to each end-face of the retention members intersects a mould splitting plane (160, 350) bisecting the container at an obtuse angle; and characterised in that the retention members are arranged into two first retention members (141, 410) and two groups (142, 420) of retention members the first retention members (141, 410) each having an end-face substantially parallel to the plane bisecting the container, and a first one of the first retention members (141, 410) and one of the groups(142, 420) is arranged on one side of the plane (160, 350), each in a respective quadrant of the container, and the other one of the first retention members (141, 410) and the other one of the groups (142, 420) is arranged on the other side of the plane (160, 350), each in a respective quadrant of the container; wherein teeth of each of the groups (142, 420) have a first inter-tooth spacing of substantially one tooth-length, and an inter-tooth spacing between an end-face of the first one of the first retention members (141, 410) and a first retention member of the group (142, 420) on the one side of the plane is larger than the first inter-tooth spacing, and wherein an inter-tooth spacing between the end-face of the first one of the first retention members (141, 410) and a first retention member of the group (142, 420) on the other side of the plane is also larger than the first inter-tooth spacing.