Usage Note: The standard rule requires that that should be used only to introduce a restrictive (or defining) relative clause, which identifies the entity being talked about; in this use it should never be preceded by a comma. Thus, in the sentence The house that Jack built has been torn down, the clause that Jack built is a restrictive clause identifying the specific house that was torn down. Similarly, in I am looking for a book that is easy to read, the restrictive clause that is easy to read tells what kind of book is desired. A related rule stipulates that which should be used with nonrestrictive (or nondefining) clauses, which give additional information about an entity that has already been identified in the context; in this use, which is always preceded by a comma. Thus, we say The students in Chemistry 101 have been complaining about the textbook, which (not that) is hard to follow. The clause which is hard to follow is nonrestrictive in that it does not indicate which text is being complained about; even if the clause were omitted, we would know that the phrase the textbook refers to the text in Chemistry 101. · Some people extend the rule and insist that, just as that should be used only in restrictive clauses, which should be used only in nonrestrictive clauses. By this thinking, which should be avoided in sentences such as I need a book which will tell me all about city gardening, where the restrictive clause which will tell me all about city gardening indicates which sort of book is needed. But this use of which with restrictive clauses is very common, even in edited prose. Moreover, in some situations which is preferable to that. Which can be especially useful where two or more relative clauses are joined by and or or: It is a philosophy in which ordinary people may find solace and which many have found reason to praise. Which may also be preferable when introducing a restrictive clause modifying a preceding phrase that contains that: We want to assign only that material which will be most helpful. · That can often be omitted in a relative clause when the subject of the clause is different from the word or phrase the clause refers to. Thus, one can say either the book that I was reading or the book I was reading. That can also be dropped when it introduces a subordinate clause: I think we should try again. That should be retained, however, when the subordinate clause begins with an adverbial phrase or anything other than the subject: She said that under no circumstances would she allow us to skip the meeting. The book argues that eventually the housing supply will increase. This last sentence would be ambiguous if that were omitted, since the adverb eventually could then be construed as modifying either argues or will increase. · There is a widespread belief, sometimes taught as correct usage, that only who and not that should be used to introduce a restrictive relative clause identifying a person. But that has been used in this way for centuries, going back to the Old English period, and has been used by the finest writers in English, as in "The man that once did sell the lion's skin / While the beast liv'd, was kill'd with hunting him" (Shakespeare). and "Scatter thou the people that delight in war" (King James Bible). In contemporary usage, who predominates in such contexts, but that is used with sufficient frequency to be considered standard, as in "The atoms in a diamond ... outnumber all the people that have ever lived or ever will" (Richard Dawkins). That also occurs idiomatically in reference to groups (where who would sound peculiar), as in "[She] had two sons, and settled into raising a family that soon included twin daughters" (David Freeman). See Usage Notes at doubt, this, whatever, which, who.