13.4.3—Geometrical (cis/trans) isomerism in alkenes,

Geometrical isomers have the same connectivity but different groups on either side of a double bond. A π bond requires sideways p-orbital overlap, so rotation would break that overlap and is restricted.

Each carbon of the C=C must have two different groups for cis/trans isomerism. “Cis” places corresponding groups on the same side; “trans” places them on opposite sides.

But-2-ene exists as cis-but-2-ene and trans-but-2-ene. But-1-ene does not give cis/trans forms because one double-bond carbon has two H atoms.

A single bond can rotate, so do not infer geometrical isomerism from any pair of substituents without checking the double-bond condition.