A transmission tower, also known as an electricity pylon or simply a pylon in British English and as a hydro tower in Canadian English, is a tall structure, usually a steel lattice tower, used to support an overhead power line. In electrical grids, they are generally used to carry high-voltage transmission lines that transport bulk electric power from generating stations to electrical substations; utility poles are used to support lower-voltage subtransmission and distribution lines that transport power from substations to electric customers. They come in a wide variety of shapes and sizes. Typical height ranges from , though the tallest are the towers of a span between the islands Jintang and Cezi in China's Zhejiang province. The longest span of any hydroelectric crossing ever built belongs to the powerline crossing of Ameralik fjord with a length of . In addition to steel, other materials may be used, including concrete and wood. There are four major categories of transmission towers: suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual effect include undergrounding. Terminology Transmission tower is the name for the structure used in the industry in the United States and some other English-speaking countries. The term electricity pylon or simply pylon comes from the basic shape of the structure, an obelisk-like structure which tapers toward the top, and the name is mostly used in the United Kingdom and parts of Europe in everyday colloquial speech. This term is used infrequently in most regions of the United States as pylon commonly refers to other things, primarily traffic cones. In the United States, the use of the term pylon is more common in the Midwest, including areas such as Cincinnati and Chicago. In Canada, the term hydro tower is commonly used as hydroelectricity constitutes a majority of the country's energy production. High voltage AC transmission towers Three-phase electric power systems are used for high voltage (66- or 69-kV and above) and extra-high voltage (110- or 115-kV and above; most often 138- or 230-kV and above in contemporary systems) AC transmission lines. In some European countries, e.g. Germany, Spain or Czech Republic, smaller lattice towers are used for medium voltage (above 10 kV) transmission lines too. The towers must be designed to carry three (or multiples of three) conductors.