Under acidic conditions, in which the carbocation rearranges fully to the most stable accessible cation, the alkene(s) that give the 2-methylbutan-2-yl cation $(\mathrm{CH}{}_3){}_2\mathrm{C}{}^{+}\mathrm{CH}{}_2\mathrm{CH}{}_3$ on protonation are:

2-Methylbut-1-ene and 2-methylbut-2-ene are protonated directly at the less substituted carbon, giving this tertiary cation. 3-Methylbut-1-ene gives a secondary cation which then undergoes a 1,2-hydride shift to the same tertiary cation. Pent-2-ene gives only straight-chain secondary cations, which cannot reach it by a simple 1,2-shift.