The study of linguistic universals is the study of the properties of any generative grammar for a natural language [...] It is useful to classify linguistic universals as 'formal' or 'substantive'. A theory of substantive universals claims that items of a particular kind in any language must be drawn from a fixed class of items. For example, Jakobson's theory of distinctive features can be interpreted as making an assertion about substantive universals with respect to the phonological component of a generative grammar. It asserts that each output of this component consists of elements that are characterized in terms of some small number of fixed, universal, phonetic features (perhaps on the order of fifteen or twenty), each of which has a substantive acoustic-articulatory characterization independent for any particular language [...] The property of having a grammar meeting a certain abstract condition might be called a 'formal' linguistic universal, if shown to be a general property of natural languages [...] formal universals involve rather the character of the rules that appear in grammars and the ways in which they can be interconnected.
- Chomsky (1969), a pag.28-29