Quaternary ammonium salt are formed by the combination of tertiary amines with alkyl halides.

${\mathrm{R}}_3\mathrm{N} + \mathrm{R}-\mathrm{X} \to {\mathrm{R}}_4{\mathrm{N}}^{+}{\mathrm{X}}^{-}$

Here, nitrogen forms three covalent bonds and one coordinate bond with the alkyl group, while the ionic bond with the halide ion.