Abstract: A barrier is fixed on a wall or other perimeter structure by way of support posts. The barrier has a support shaft along which a number of rotatable units are mounted. Each rotatable unit is formed from two interengaged elements, each element having been formed from a length of metal tubing. Part of the tubing has been cut longitudinally to define blades extending from one end of a tubular portion, and splayed outwardly and spaced apart. Two such elements are assembled so that their blades interengage to form one of the units. The rotatable units so constructed are simple to manufacture, but provide an effective barrier.