Sketch-based user interfaces are increasingly common and are being built for a variety of different disciplines. However, at present the implementation of sketch recognizers is quite time consuming since they are mostly based on specific techniques, as opposed to several other fields such as textual/visual languages and speech recognition, which benefit from the availability of compiler generation techniques and tools. This paper proposes a grammar formalism, namely Sketch Grammars (SkGs), for describing both the shape of the symbols? language and the syntax of sketch languages. Recognizers are automatically generated from the sketch grammar descriptions.