Is it correct to use a comma before the relative pronoun "that"?

[sitifan]

musk ox: a large animal with long brown or black hair and curved horns, that lives in northern Canada and Greenland (Longman Advanced American Dictionary, page 960)

Is it correct to use a comma before the relative pronoun "that"?

 

[sitifan]

1. piston: a part of an engine consisting of a short solid piece of metal inside a tube, which moves up and down to make the other parts of the engine move (Longman Dictionary of Contemporary English)

2. piston: a part of an engine consisting of a short solid piece of metal inside a tube, that moves up and down to make the other parts of the engine move (Longman Advanced American Dictionary)

#1(British English) and #2(American English) are identical except for the relative pronouns.

 

[Charlie Bernstein]

In those definitions, there should be no commas before the two thats.

The comma before which is correct.

 

[PaulMatthews]

musk ox: a large animal with long brown or black hair and curved horns, that lives in northern Canada and Greenland (Longman Advanced American Dictionary, page 960)

Is it correct to use a comma before the relative pronoun "that"?

Not only is it wrong to use a comma with a defining relative, but it's also wrong to treat "that" as a relative pronoun when in fact it's a subordinator -- the same subordinator that introduces declarative content clauses such as, for example, "I know that he is a teacher".

 

[PaulMatthews]

1. The musk ox is a large animal which lives in northern Canada.
2. The musk ox is a large animal that lives in northern Canada.
3. I know that this is a musk ox.

Are you saying that the 'which' in #1 is a relative pronoun but the 'that' in #2 is not - it is, like the 'that' in #3, a subordinator?

Yes: in terms of word class (POS) and function, that's exactly what I'm saying.

"That" is simply a subordinator functioning as a marker.