Editing Allenes (section)

==Delta convention==
Many rings or ring systems are known by semisystematic names that assume a maximum number of noncumulative bonds. To unambiguously specify derivatives that include cumulated bonds (and hence fewer hydrogen atoms than would be expected from the skeleton), a lowercase delta may be used with a subscript indicating the number of cumulated double bonds from that atom, e.g. 8δ<sup>2</sup>-benzocyclononene. This may be combined with the λ-convention for specifying nonstandard valency states, e.g. 2λ<sup>4</sup>δ<sup>2</sup>,5λ<sup>4</sup>δ<sup>2</sup>-thieno[3,4-c]thiophene.<ref>{{cite journal|url=http://www.chem.qmul.ac.uk/iupac/hetero/De.html|title=Nomenclature for cyclic organic compounds with contiguous formal double bonds (the δ-convention)|journal=Pure Appl. Chem.|volume=60|page=1395-1401|year=1988|doi=10.1351/pac198860091395|s2cid=97759274|doi-access=free}}</ref>