A container (100), comprising a thread (120) arranged around an opening (110) for engaging a corresponding thread (120) of a closure (400), more than four retention members (140) arranged around the opening (110), each retention member (140) having an end-face for abutting a member (510) on an interior surface of the closure (400) to impede unthreading of the closure (400), the retention members (140) being arranged such that a tangent (170) to each end-face of the retention members (140) intersects a plane (160, 350) bisecting the container (100) at an obtuse angle (171), wherein the retention members (140) are arranged in first and second groups, the first group (141, 410) comprising one retention member (140) having an end-face substantially parallel to the plane bisecting the container (100), and the second group (142, 420) comprising two or more retention members (140), the first and second groups being arranged on one side of the plane (160, 350), each in a respective quadrant of the container (100), the teeth of the second group (142, 420) have a first inter-tooth spacing, and a second inter-tooth spacing is provided between an end-face of the retention member of the first group (141, 410) and a first retention member of the second group (142, 420), and the second inter-tooth spacing is larger than the first inter-tooth spacing.