Every strongly open map is a relatively open map and moreover, because formula_10 is always an open subset of formula_3 the image formula_38 of a strongly open map formula_1 is necessarily an open subset of the codomain formula_5 However, a relatively open map formula_1 is a strongly open map if and only if its image formula_42 is an open subset of its codomain formula_5 Because of this simple characterization, it is often straightforward to apply results involving one of these two definitions of "open map" to a situation involving the other definition.