1 The word that can be omitted in standard English where it introduces a subordinate clause, as in she said (that) she was satisfied. It can also be dropped in a relative clause where the subject of the subordinate clause is not the same as the subject of the main clause, as in the book (that) I’ve just written (‘the book’ and ‘I’ are two different subjects). Where the subject of the subordinate clause and the main clause are the same, use of the word that is obligatory, as in the woman that owns the place (‘the woman’ is the subject of both clauses). 2 It is sometimes argued that, in relative clauses, that should be used for non-human references, while who should be used for human references: a house that overlooks the park but the woman who lives next door. In practice, while it is true to say that who is restricted to human references, the function of that is flexible. It has been used for human and non-human references since at least the 11th century, and is invaluable where both a person and a thing is being referred to, as in a person or thing that is believed to bring bad luck. 3 Is there any difference between the use of that and which in sentences such as any book that gets children reading is worth having, and any book which gets children reading is worth having? The general rule in British English is that, in restrictive relative clauses, where the relative clause serves to define or restrict the reference to the particular one described, which can replace that. However, in non-restrictive relative clauses, where the relative clause serves only to give additional information, that cannot be used: this book, which is set in the last century, is very popular with teenagers but not this book, that is set in the last century, is very popular with teenagers. In US English which is generally used only for non-restrictive relative clauses.