Chlorine replaces two hydrogen atoms of toluene (on the ring and/or on the methyl group), giving $\mathrm{C}{}_7\mathrm{H}{}_6\mathrm{Cl}{}_2$ with the benzene ring retained. How many structural isomers are possible?

Both Cl on the ring: 6 dichlorotoluenes (2,3-, 2,4-, 2,5-, 2,6-, 3,4-, 3,5-). One Cl on the ring and one on CH3: 3 (o-, m-, p-chlorobenzyl chloride). Both Cl on CH3: benzal chloride, 1. Total 6 + 3 + 1 = 10, matching exhaustive H-pair substitution.