It is also important to note that trans-alkenes are often more stable than cisalkenes due to diminished steric hindrance (p. 232), but this is not always the case. It is known, for example, that cis-1,2-difluoroethene is thermodynamically more stable than trans-1,2-difluoroethene. This appears to be due to delocalization of halogen lone-pair electrons and an antiperiplanar effect between vicinal antiperiplanar bonds.