In the public's mind, concrete is synonymous with rock-like density. In reality, however, walls of concrete which are to experience water pressure on one side must be specially constructed if they are to remain watertight. Even the most dense concrete will allow a certain amount of water under pressure to pass through. However, the real trouble lies in the leakage which can occur though the joints which are a necessity in practically all concrete walls. This problem is common in such constructions as basement walls, swimming pools, power plants, water tanks and canal structures. Although each has claims for its individual superiority, almost all waterstops conform to one of six shapes: labyrinth, flat corrugated, dumbbell 2-bulb, dumbbell 3-bell, flexible metal and rigid metal plate. Labyrinth waterstops were evolved to avoid carpentry in splitting forms. Dumbbell varieties rely upon the bulbs at the ends to anchor the unit securely. Flexible metal waterstops are V-shaped at the joint to allow movement. Rigid metal plates are embedded uncoated in the concrete. Materials used for waterstpos are either metal (copper or stainless steel), rubber or plasticized polyvinyl chloride plastics. When choosing a water stop look for water tightness and durability.