The development of the Old French (OF) system of one or two cases and three declensions from the Classical Latin system of six cases and five declensions, via Vulgar Latin (VL), cannot be accounted for without reference to the syntax and semantics / pragmatics of the central grammatical relations of subject, object, attribute. It is argued in this paper that many VL/OF ‘analogical’ inflectional changes and re-distributions of nouns among existing or newly formed declension classes, changes that seem arbitrary from a purely morphological or phonological perspective, are determined by a general functional principle: overt relational distinctions are made (created or not abandoned) only if necessary in order to distinguish co-occurring term phrases and to identify their grammatical relation / semantic role, in so far as this is not possible on account of the pragmatic and semantic properties of these terms. This investigation also has implications with regard to the relationship between functionally based and grammaticalized relational encoding. Although the inflectional coding of each noun may be strictly grammaticalized rather than functionally variable, languages with declension classes do not a priori fall outside the scope of the functional principle; rather, it is this kind of nominal classification itself which has a functional rationale.