We define a pattern structure whose objects are elements of a supporting ontology. In this framework, descriptions constitute trees, made of triples subject-predicate-object, and for which we provide a meaningful similarity operator. The specificity of the descriptions depends on a hy-perparameter corresponding to their depth. This formalism is compatible with ontologies formulated in the language of RDF and RDFS and aims to set up a framework based on pattern structures for knowledge discovery in the web of data.