In this paper we present a framework of syntactic models for the definition and implementation of visual languages. We analyze a wide range of existing visual languages and, for each of them, we propose a characterization according to a syntactic model. The framework has been implemented in the Visual Language Compiler-Compiler (VLCC) system. VLCC is a practical, flexible and extensible tool for the automatic generation of visual programming environments which allows to implement visual languages once they are modeled according to a syntactic model.