What vs. which

[Allen165]

On the site I linked to below, someone asked:

"I would like to know when we should say what and when we should say which. Why do you use what and which in this example instead of only what?

• It is difficult to know what idioms are in fashion and which are not."

The answer she got was that "what" and "which" were interchangeable in the sentence above. I'm not sure I agree. "It is difficult to know what idioms are in fashion and what are not" doesn't sound right to me.

What do you think?

Learning English | BBC World Service

Thanks!

[Mehrgan]

On the site I linked to below, someone asked:


"I would like to know when we should say what and when we should say which. Why do you use what and which in this example instead of only what?

• It is difficult to know what idioms are in fashion and which are not."

The answer she got was that "what" and "which" were interchangeable in the sentence above. I'm not sure I agree. "It is difficult to know what idioms are in fashion and what are not" doesn't sound right to me.

What do you think?

Learning English | BBC World Service

Thanks!

***not a teacher***
'Which' reaplaces 'what idioms', and not only 'what'. So I suppose 'what' cannot replace 'which' on its own.

[Khosro]

I took back my reply Jasmin165 ! You could also take back your thank.

[Bamako7]

***not a teacher***
'Which' reaplaces 'what idioms', and not only 'what'. So I suppose 'what' cannot replace 'which' on its own.

I vehemently disagree.


It is difficult to know what idioms are in fashion and which (idioms) are not.

It is difficult to know what idioms are in fashion and what (idioms) are not.

It is difficult to know which idioms are in fashion and which (idioms) are not.

The second "idioms" is substituted for an empty proform.
We use "which" to refer to a closed set, and "what" to refer to an open set. "what idioms" is reference to an open set, "which (idioms)" is reference to a closed set. I trust you can see the inconsistency there.

[Allen165]

So what did the author mean by "interchangeable"?

Thanks!

[5jj]

So what did the author mean by "interchangeable"?

Bamako is largely right, though when he writes "we use...", he appears to be stating a fact. However, while those who consider themselves to be 'careful' speakers do indeed prefer to use 'which' when they are thinking of a limited number of choices and 'what' when they are not, many speakers do not observe this difference. For such speakers the words are interchangeable, which is what your writer was suggesting.

So, in the sentence below:

It is difficult to know .... idioms are in fashion and .... (idioms) are not

we can use either 'what' or 'which' with no real difference in meaning. Bamoko suggested that it would be inconsistent to use 'what' in one gap and 'which' in the other. He is right, but some people will still do it. It is not elegant, but it does not cause any problems in understanding, and many native speakers would not notice anything wrong.

[MrPedantic]

On another site, I saw the compact advice that "what identifies; which selects".

This corresponds to B7's comment on closed and open sets.

A rule-of-thumb of this kind may possibly help non-native speakers to choose which word to use. However, as 5jj intimates, it will not necessarily help you to divine a native speaker's intended meaning, since not all native speakers observe a difference there.

Luckily, it's unlikely to be an important distinction, in most contexts.

Best wishes,

MrP

[birdeen's call]

Do you consider the following rule correct? (I've just made it up.)

"What" identifies by features. "Which" identifies by entities.

[5jj]

Do you consider the following rule correct? (I've just made it up.)

"What" identifies by features. "Which" identifies by entities.

Personally, I find MrP's "what identifies; which selects" simpler.