Why is z-stilbene not planer?

Z-stilbene is not planar due to the presence of substituents that introduce steric hindrance and torsional strain. The cis configuration of the double bond causes the phenyl rings to be oriented in such a way that they cannot lie flat in the same plane, leading to a twist in the molecular structure. This non-planarity can affect its physical and chemical properties, such as its reactivity and interaction with light.