Polyisoprene is, strictly speaking, a collective name for polymers that are produced by polymerization of isoprene. In practice polyisoprene is commonly used to refer to synthetic cis-1,4-polyisoprene, made by the industrial polymerisation of isoprene. Natural forms of polyisoprene are also used in substantial quantities, the most important being "natural rubber", which is derived from the sap of trees. Both synthetic polyisoprene and natural rubber are highly elastic and consequently used in a variety of applications, such as making tires.