# Why do E and Z isomers occur?

For even a simple olefin such as $\text{2-butylene}$, geometric isomerism can occur because of the $C = C$ bond. The methyl groups can be on the same side of the $C = C$ bond to give the $\text{cis}$ or $\text{Z}$ isomer, or on opposite sides of the $C = C$ bond to give the $\text{trans}$ or $\text{E}$ isomer.
Both isomers have the same $C - C$ connectivity: $C 1$ connects to $C 2$......to......$C 4$, yet they are manifestly different geometrically. They are thus diastereomeric, and should give rise to different chemical and physical properties, and indeed they do.