A clause is a sentence or sentence-like linguistic construction included within another sentence. A main clause might be a simple noun plus verb ('I did it'). A co-ordinate clause is of equal status with the main clause: 'I did it and she did it at the same time.' A subordinate clause might be nested within a sentence using the conjunction 'that': 'he said that the world was flat.' Here 'he said' is the main clause and the subordinate clause is 'the world was flat'. Relative clauses are usually introduced by a relative pronoun: 'I read the book which was falling to pieces'; 'She spoke to the man who was standing at the bar.'