It is useful to classify linguistic universals as 'formal' or 'substantive' [...] 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.29

[…] if we conclude that the transformational cycle is a universal feature of phonological component, it is unnecessary, in the grammar of English, to describe the manner of functioning of those phonological rules that involve syntactic structure. This description will now have been abstracted from the grammar of English and stated as a formal linguistic universal, as part of the theory of generative grammar.
- Chomsky (1969), a pag.35-36