Comma usage of non-restrictive and restrictive clauses containing that and which

Question

I am a native speaker of English but am having trouble with the distinction between that and which, with respect to restrictive and non-restrictive clauses and comma usage.

For example, which is correct:

Dragons, that breath fire, are scary.

or

Dragons that breath fire are scary.

or

Dragons, which breath fire, are scary.

Since all dragons breath fire, breath fire seems to be descriptive or non-restrictive, so there should be no commas used.

My punctuation has never been that spot-on, although I don't have much trouble with other areas of grammar and syntax. I feel that punctuation heavily affects the readability of my writing even if the word-ordering of the sentence is grammatically correct. The links I've found about this have still left me slightly confused.

Cheers!

Edit: Upon some further reading, it seems that and which are technically interchangeable; however, that usually proceeds a restrictive clause, and which usually proceeds a non-restrictive clause. Please correct me if I'm wrong about this.

Reference:

Extended Rules for Using Commas

Answer 1

It depends quite a lot on what you are trying to say about dragons, or whatever. If there were two sorts of dragon, one sort fire-breathing, the other not, you could express your feeling that the former are scary with a defining phrase, thus:"dragons that breathe fire are scary". No commas needed; the first four words are a single thought. If the fire-breathing were merely incidental to your being scared of dragons, which, as you say, are in reality(!) all fire-breathing, but you also wished to remind us of that fact, you would write: "dragons, which breathe fire, are scary". The use of the comma tells the reader that the fire-breathing is incidental to the scariness.

That is one convention. I think it is logical and helpful to the reader. Be aware that some writers would nevertheless use commas in both cases. In my experience they have been American English speakers possibly influenced by German practice, which always uses commas to separate such phrases.

David Crystal, in his "A Little Book of Language" writes: 'There is a strong personal element in the way people punctuate their writing' and 'Some people get very upset about such things'.

Answer 2

Without a comma, it would be a restrictive relative clause—a relative clause that identifies the referent rather than describes it.

So Dragons that breath fire are scary would mean that only dragons that breath fire are scary; dragons that don't aren't.

Dragons, which/^?that breath fire, are scary is a non-restrictive relative clause as evident from the presence of the comma. It implies that all dragons are scary because they breath fire.

As for the relative pronoun, which is favored in the non-restrictive relative clause; that is marginally acceptable and therefore should be avoided in formal writing.