The best Williamson synthesis of tert-butyl methyl ether, $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{O}-\mathrm{CH}{}_3$, uses

The Williamson reaction is an SN2 reaction of an alkoxide on an alkyl halide, so the halide should be methyl or primary. A tertiary halide with a basic alkoxide undergoes E2 and gives isobutene. Hence the bulky alkoxide is combined with methyl iodide.