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', (mostly cis-1,4-polyisoprene), which is derived from the sap of trees. Both synthetic polyisoprene and natural rubber are highly elastic and consequently used to make tyres and a variety of other applications.