A cumulene is a compound having three or more cumulative (consecutive) double bonds. They are analogous to allenes, such as propadiene, but with at least one additional alkene unit. The simplest molecule in this class is thus butatriene, which is also called simply cumulene. Unlike most alkanes and alkenes, cumulenes tend to be rigid, comparable to polyynes. Cumulene carbenes H2Cn for n from 3 to 6 have been observed in interstellar molecular clouds and in laboratory experiments by using microwave and infrared spectroscopy. Cumulenes containing heteroatoms are called heterocumulenes; an example is carbon suboxide.