 Sentences can consist of any number of clauses. If a sentence consists of one clause we call it a simple sentence such as the cat sat on the mat. Thus the terms clause and sentence can be used synonymously here. If there is more than one clause in a sentence we refer to the sentence as multiple sentence. The relationship between the clauses in multiple sentences can be manifested in two ways. In compound sentences such as the cat sat and the dog ran the clauses are coordinated with each other by means of a coordinator. The most familiar coordinators that are used to coordinate clauses are the conjunctions and but and or. Complex sentences such as the cat sat when the dog ran by contrast consist of a main clause and one plus any number of subordinate clauses. The connection between the clauses is often signaled by means of subordinating conjunctions such as when, although, because, before, if and so on. In many cases especially in written English multiple sentences often combine coordinate and subordinate structures. In when Robert finally hung up Bill went downstairs finding Mary on the couch we have a multiple declarative sentence. It exhibits two clauses in a compound relationship without an overt coordinator where clause two is an elliptical non-finite clause. The missing element is Bill. Clause one is a complex sentence with an adverbial subordinate clause of time signaled by the initial subordinating conjunction when and a finite main clause.