What is a double replacement equation composed of?

A double replacement equation has two reactants, each composed of two species, and they exchange "partners" to form either a gas, a precipitate, or a liquid (H2O most often). If none of these happen, and all reactants and products are soluble, then NO REACTION takes place. Example:AB(aq) + CD(aq) ==> AD(s) + CB(aq)