This paper is concerned with (average)X-syntax which explores the nature of the phrase structure rules of English on the basis of a set of observations concerned with subordinating adverbial conjunctions functioning as complementizers of restrictive or non-restrictive adverbial clauses. The paper shows first that basically the (average)X-framework of the phrase structure makes the right sort of distinctions to capture the descriptive observations of subordinating conjunctions. However, various modifications of the framework are needed in order to describe the phenomena discussed.