Consider the following acid–base reaction:
Acetic acid, , is an acid because it donates a proton to water () and becomes its conjugate base, the acetate ion ().  is a base because it accepts a proton from  and becomes its conjugate acid, the hydronium ion, ().
The reverse of an acid–base reaction is also an acid–base reaction, between the conjugate acid of the base in the first reaction and the conjugate base of the acid. In the above example, ethanoate is the base of the reverse reaction and hydronium ion is the acid.
One feature of the Brønsted–Lowry theory in contrast to Arrhenius theory is that it does not require an acid to dissociate.