We introduce the notion of a differentiation function of acontext-free grammar which gives the number of terminal words that can bederived in a certain number of steps. A grammar is called narrow (ork-narrow) iff its differentiation function is bounded by a constant(by k). We present the basic properties of differentiation functions,especially we relate them to structure function of context-free languagesand narrow grammars to slender languages. We discuss the decidability of the equivalence of grammars with respect to the differentiation function andstructure function and prove the decidability of the k-narrowness of context-free grammars. Furthermore, we introduce languages representing the graph of the differentiation and structure function and relate these languages to those of the Chomsky hierarchy.