by poles, but by metal pylons (known as transmission towers in the US). For economic or practical reasons, such as to save space in urban areas, a distribution line is often carried on the same poles as a sub transmission line but mounted under the higher voltage lines; a practice called "underbuild". Telecommunication cables are usually carried on the same poles that support power lines; poles shared in this fashion are known as joint-use poles, but may have their own dedicated poles. Description The standard utility pole in the United States is about long and is buried about in the ground. However, poles can reach heights of or more to satisfy clearance requirements. They are typically spaced about apart in urban areas, or about in rural areas, but distances vary widely based on terrain. Joint-use poles are usually owned by one utility, which leases space on it for other cables. In the United States, the National Electrical Safety Code, published by the Institute of Electrical and Electronics Engineers (IEEE) (not to be confused with the National Electrical Code published by the National Fire Protection Association [NFPA]), sets the standards for construction and maintenance of utility poles and their equipment. Pole materials Most utility poles are made of wood, pressure-treated with some type of preservative for protection against rot, fungi and insects. Southern yellow pine is the most widely used species in the United States; however, many species of long straight trees are used to make utility poles, including Douglas fir, jack pine, lodgepole pine, western red cedar, and Pacific silver fir. Traditionally, the preservative used was creosote, but due to environmental concerns, alternatives such as pentachlorophenol, copper naphthenate and borates are becoming widespread in the United States. In the United States, standards for wood preservative materials and wood preservation processes, along with test criteria, are set by ANSI, ASTM, and American Wood Protection Association (AWPA) specifications. Despite the preservatives, wood poles decay and have a life of approximately 25 to 50 years depending on climate and soil conditions, therefore requiring regular inspection and remedial preservative treatments. Woodpecker damage to wood poles is the most significant cause of pole deterioration in the U.S. Other common utility pole materials are steel and concrete, with composites (such as fiberglass) also becoming more prevalent. One particular patented utility pole variant used in Australia is the Stobie pole, made up of two vertical steel posts