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I am confused between "which" and "that". I don't know in above sentences which is correct. According to me, both sentences are right. I have read some posts on the usages of "which" and "that", but I didn't get my answer. Which of these sentences is correct, and why?

1) Animals which lay eggs are called birds.

2) Animals that lay eggs are called birds.

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    Both forms of the claim are factually incorrect: most fish, amphibians, reptiles, and even certain mammals (the monotremes) also lay eggs. – Mico Dec 27 '14 at 12:34
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    Made an account just to see if anyone had made the previous comment. You can also add almost all invertebrates (insects, crustaceans, molluscs, etc) to the list above. – March Ho Dec 28 '14 at 8:55
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    In fact animals that lay eggs are called "oviparous animals". This reminds me of a famous "dad joke": which is correct, "the yolk of an egg is white" or "the yolk of an egg are white"? The child of course says "the yolk of an egg is white", at which point you tell them that no, it is yellow, ha ha ha ha! – Eric Lippert Dec 28 '14 at 16:54
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There are two types of relative clauses in English, which I shall call 'defining clauses' and 'commenting clauses'. They are best described with an example:

Pilots who have dull minds seldom live long

Pilots, who have dull minds, seldom live long.

The first sentence is a warning about the dangers of having a dull mind if you want to be a pilot. The second is insulting to all pilots.

Defining clauses are never separated from the main sentence by a comma: commenting clauses always are.

There's an old fashioned rule that you should always use 'that' rather than 'which' when you are writing a defining clause. Thus

Animals that lay eggs are called birds.

is grammatical, while

Animals which lay eggs are called birds.

is ungrammatical. These days, however, prescriptivism (language rules) is unfashionable and people are far more likely to consider both sentences to be grammatical, and to mean the same thing. Moreover, in colloquial speech, and even in writing, the rule has never been universally followed.

Of course, neither sentence is factually correct: a snake lays eggs, but is not called a bird!

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    Part of my job entails writing technical standards in the RFC ("Request For Comment") series, and the editors are particularly strict on "that vs which". It's not seen as being "old fashioned". – Alnitak Dec 27 '14 at 21:35
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    @Alnitak There are style guides and then there are style guides. Perhaps you could explain more about your opinion in an answer post? – F.E. Dec 28 '14 at 0:17
  • @Donkey_2009 I call the snake a bird, thus making the claim correct. – ikdc Dec 28 '14 at 0:26
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    @Alnitak People who hold old-fashioned beliefs rarely describe their own beliefs as "old-fashioned." Where a style guide is mandated, however, it is irrelevant whether it is old-fashioned or merely strict; you are well-advised to follow it anyway. – David K Dec 28 '14 at 15:59
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See this Oxford Dictionaries article for an understanding of restrictive and non-restrictive relative clauses. Oxford says:

That and which introduce what’s known as a restrictive relative clause. This is a clause containing essential information about the noun that comes before it. If you leave out this type of clause, the meaning of the sentence is affected – indeed, it will probably not make much sense at all. Restrictive relative clauses can be introduced by that, which, whose, who, or whom.

The other type of relative clause is known as a non-restrictive relative clause. This kind of clause contains extra information that could be left out of the sentence without affecting the meaning or structure. Non-restrictive clauses can be introduced by which, whose, who, or whom, but you should never use that to introduce them.

By the way, if we take your sentence "Animals that lay eggs are called birds" and make a syllogism, we can then say "Fish lay eggs," and so "Fish are called birds." A small problem.

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    Websites get rearranged all the time, which breaks links. Please at least give a brief summary of the page you've linked so your answer will still have some content if Oxford Dictionaries break the link. – David Richerby Dec 27 '14 at 10:44
5

Both sentences are grammatically correct.

In this construction, which and that both serve the same purpose. They are both relative pronouns. There is no difference in meaning. The words are interchangeable.

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    incorrect IMHO. The first sentence is grammatically correct, whilst the second is not because it is missing the comma that separates the commenting clause from the subject. Neither is actually technically correct, of course! :) – Alnitak Dec 27 '14 at 21:31
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    @Alnitak Could you perhaps provide a vetted grammar source for your opinion that the "which" version is not correct? Maybe you could do that in an answer post? – F.E. Dec 28 '14 at 0:15
  • I don't have a vetted source - it's just what I've been taught (albeit affirmed by numerous unofficial sources on the net) – Alnitak Dec 28 '14 at 1:17
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I think the OP is confused whether he should use the relative pronoun "that" or "which" in the sentence.

In fact, "that/which lay eggs" is a defining relative clause in the sentence, which is used without putting a comma before and after it. It tells which animals you are talking about. Without this clause, the sentence doesn't make sense. As for relative pronoun that/which, you can use either "that" or "which" for things/animals in the defining relative clause. However, the use of "which" is a bit formal and the use of "that" is common in informal English.

So both sentences are grammatically correct.

  • There are some people who say the sentence with "which" is ungrammatical; and in certain contexts, such as technical writing, I would carefully avoid it, though I would find it perfectly acceptable in many other contexts. I think that makes the use of "which" less formal, not more. – David K Dec 28 '14 at 14:49
  • @David K, I agree. – Khan Dec 28 '14 at 18:56

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