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An allene contains the unit $\mathrm{C}=\mathrm{C}=\mathrm{C}$ with two consecutive carbon–carbon covalent bonds of order two. Suitable terminal substituents give axial chirality.

An allene has a $\mathrm{C}=\mathrm{C}=\mathrm{C}$ unit whose terminal substituents lie in perpendicular planes. In $\mathrm{abC}=\mathrm{C}=\mathrm{Ccd}$, with $a\ne b$ and $c\ne d$, the two terminal pairs can define enantiomers without any tetrahedral carbon stereocenter. The groups need be different within each end pair, not all four mutually different. This is axial chirality.^[ref]