Improvisation in North Indian tabla drumming is similar to speech insofar as it is bound to an underlying system of rules determining correct sequences. The parallel is further reinforced by the fact that tabla music may be represented with an oral notation system used for its transmission and, occasionally, performance. Yet the rules are implicit and available only through the musicians' ability to play correct sequences and recognise incorrect ones. A linguistic model of tabla improvisation and evaluation derived from pattern languages and formal grammars has been implemented in the Bol Processor, a software system used in interactive fieldwork with expert musicians. The paper demonstrates the ability of the model to handle complex structures by taking real examples from the repertoire. It also questions the relevance of attempting to model irregularities encountered in actual performance.