We can consider a pushdown automaton as a word rewriting system with labelled rules applied only in a prefix way. The notion of context-free graph, defined by Muller and Schupp is then extended to the notion of prefix transition graph of a word rewriting system. Prefix transition graphs are context-free graphs, and we show they are also the rooted pattern graphs of finite degree, where a pattern graph produced from a finite graph by iterating the addition of a finite family of finite graphs (the patterns). Furthermore, this characterisation is effective in the following sense : any finite family of patterns generating a graph G having a finite degree and a root, is mapped effectively into a rewriting system R on words such that the prefix transition graph of R is isomorphic to G, and the reverse transformation is effective.