Stilbene exists as two possible stereoisomers. One is trans-1,2-diphenylethylene, called (E)-stilbene or trans-stilbene. The second is cis-1,2-diphenylethylene, called (Z)-stilbene or cis-stilbene, and is sterically hindered and less stable because the steric interactions force the aromatic rings out-of-plane and prevent conjugation. Cis-stilbene is a liquid at room temperature (melting point: ), while trans-stilbene is a crystalline solid which does not melt until around , illustrating the two isomers have significantly different physical properties.