The article revisits a proposal by Luís J. Prieto for the (re)definition of syntax, dated 1988-1989. Prieto’s idea deserves to be reclaimed for multiple reasons: the resemblance of what he calls “phonological syntax” with what is called today “prosodic phonology”; the way in which it helps to understand the Hjelmslevian principle of biplanarity (heteroplane function); and, most of all, its semiotic value, which goes beyond the strictly linguistic level and makes it a fundamental notion for a semiotic theory of knowledge.