obligatory vs optional [Lat. obligatio ‘the state of being legally etc. liable,’ optio ‘choice’]

Property of rules which specifies the conditions of their application. The distinction applies to all levels of description and has an important role in the syntactic and semantic description of sentence structure based on verb valence. In contrast to valenceindependent elements in a sentence, such as free adjuncts and attributes, actants which depend on valence can be divided into two groups: valence positions which must be filled and those which do not need to be filled under certain conditions (i.e. optional); cf. Caroline is writing a letter to her mother vs Caroline is writing (something), but Caroline gave her mother a present vs *Caroline gave her mother. This structurally based distinction refers exclusively to grammatical completeness or wellformedness; it does not always correspond to semantic-pragmatic factors such as completeness and differentiation of information. For other uses of this distinction, also free variation and transformations.