xsd:string One linguistic unit, Sign_1, has a relation to another linguistic unit, Sign_2. This relation may be semantic, phonological, orthographic, or distributional in nature. The parameter along which the two signs are set in relation to each other is incorporated in the meaning of the target words in this frame. When the relation is symmetrical, the two linguistic units can be expressed jointly, Signs. Otherwise they have to be expressed disjointly. Ordinarily it is assumed that one linguistic unit has exactly one meaning but in cases where this is not so, a set of Circumstances may be specified that helps pick out the relevant sense. Happy and sad are antonyms. In one sense, dog is a hyponym of itself.