This paper investigates the syntax and diachrony of Italian causal clauses introduced by perché, siccome, and poiché. Although often treated as near-synonyms in Contemporary Italian, these subordinators differ systematically in their syntactic distribution, interpretive properties, and diachronic development. We show that perché introduces central adverbial clauses, merged within the vP/TP domain, whereas siccome and poiché introduce peripheral adverbial clauses, merged in the left periphery. This structural split correlates with a cluster of diagnostics: only perché-clauses can occur within the scope of matrix focus, negation, or epistemic operators, and only they can function as fragment answers. Conversely, siccome- and poiché-clauses consistently outscope matrix operators and encode non-at-issue content. A diachronic study reveals that the internal and external syntax of causal clauses introduced by each subordinator has remained stable from Old Italian to the present. However, siccome- and poiché-clauses display different semantics, as they derive from non-causal constructions (they originate from comparative and temporal clauses, respectively). We argue that the contrasting behaviors follow from the structural composition of the subordinators.