The first characterizations of concepts are the expressions that are used to refer to them in natural language: their labels. SKOS provides three properties to attach labels to conceptual resources: skos:prefLabel, skos:altLabel and skos:hiddenLabel. Each property implies a specific status for the label it introduces, ranging from a strong, univocal denotation relationship, to a string to aid in lookup. These properties are formally defined as being pairwise disjoint. This means, for example, that it is an error if a concept has a same literal both as its preferred label and as an alternative label.