A water block is the watercooling equivalent of a heatsink. It can be use on many different computer components including the central processing unit (CPU), GPU, and Northbridge chipset on the motherboard. It consists of at least two main parts, the "base", which is the part that makes contact with the device being cooled, usually made of a high conductivity metal such as aluminum or copper and in some cases silver as is found in many newer blocks. The second part, the "top" ensures the water is contained safely inside the "block" and has connections that allow hosing to connect it with the water cooling loop. The top can be made of the same metal as the base, transparent Perspex, Delrin, Nylon, or HDPE. Most newer high end water blocks also contain mid-plates which serve to add jet tubes, nozzles, and other flow altering devices.