Relative clauses are usually divided into two types: defining (restrictive) and non-defining (non-restrictive). Since the definition tries to narrow the meaning of an item, defining relative clauses are used in formal definitions, specifying the item. Non-defining clauses provide extra information and are used to expand a definition. In academic writing, it is common to use reduced relative clauses because they are as clear as non-reduced relative clauses, but are shorter and more economical with words.