Relative clauses contain information about nouns. There are two main types: defining
The form and usage of each is quite different.
Have you seen the book that I was reading this morning?
This is a defining relative clause and contains information about the noun book
The River Nile, which is over 6,500 kilometres long, is Egypt's main source of water.
This is a non-defining relative clause and contains information about the noun the River Nile
The information in the first example, that I was reading this morning
, identifies the book - it tells us which book. The information in the second example, which is over 6,500 kilometres long
is extra information.
The main differences between the two types of clauses are summarized below.