For cis-trans isomerism in cyclic compounds, each carbon of the ring must bear two DIFFERENT substituents. 1,1-Dimethylcyclopropane: one carbon has two identical methyls → no cis-trans. 1,2-Dimethylcyclohexane: each of C1 and C2 has one methyl and one H → different groups on each → cis-trans isomers exist ✓. Pent-1-ene (CH\(_2\)=CHCH\(_2\)CH\(_2\)CH\(_3\)): terminal alkene, one end has two H → no isomerism. 2-Methylhex-2-ene: one end has two methyls → no cis-trans.