A clause typically contains at least a subject noun phrase and a finite verb. While the subject is usually a noun phrase, other kinds of phrases (such as gerund phrases) work as well, and some languages allow subjects to be omitted. There are two types of clauses: independent and subordinate (dependent). An independent clause demonstrates a complete thought; it is a complete sentence: for example, I am sad. A subordinate clause is not a complete sentence: for example, because I have no friends. See also copula for the consequences of the verb to be on the theory of sentence structure.