Acetaldehyde is warmed with excess methanol containing a little dry HCl gas. The product is

Dry acid catalyses addition of two molecules of methanol to the aldehyde; the first gives the hemiacetal and the second replaces the OH, giving the acetal $\mathrm{CH}{}_3\mathrm{CH}(\mathrm{OCH}{}_3){}_2$ (1,1-dimethoxyethane). The hemiacetal is only an intermediate.