A cycloalkene or cycloolefin is a type of alkenehydrocarbon which contains a closed ring of carbonatoms, but has no aromatic character. Some cycloalkenes, such as cyclobutene and cyclopentene, can be used as monomers to produce polymer chains. Due to geometrical considerations, smaller cycloalkenes are almost always the cis isomers, and the term cis tends to be omitted from the names. In larger rings (from around 8 atoms), cis–trans isomerism of the double bond may occur.