For example,

We have a conversation on food.
We have a conversation about food.

  • I've heard more people using 'about' for this sentence rather that 'on'. Look at this Ngram – Varun Nair Mar 23 '16 at 10:52
  • I would say "about + topic" is preferred in colloquial speech, "on + topic" is a bit more formal, often found in written speech. – rogermue Mar 23 '16 at 13:49
  • @rogermue: I would say that there is much more than formality at play here; see my answer. I would agree though that informal speech would tend to use "about" as it is by default correct and the speaker would not need to know the finer detail of the English language to know whether "on" is appropriate or not. – user21820 Mar 23 '16 at 16:17

There is no such difference I find. As I see various examples, both are common and valid.

For instance, Merriam Webster lists many examples that use 'about' and COCA search renders many examples with 'conversation 'on''.

Consider Varun's comment. Ngram says that 'about' is more common than 'on'.

  • It is not true that there is no difference. To the non-native speaker it indeed has no difference, but to the native ear not only do they sound different but in some cases "on" cannot be used or sounds terrible. See my answer for examples. – user21820 Mar 23 '16 at 15:55
  • @user21820 Of course there are differences between on and about, but what difference is there in this specific sentence? Your answer doesn't explain that. – Gilles 'SO- stop being evil' Mar 23 '16 at 16:08
  • @Gilles: I did explain that the connotations of the prepositions are different, and it is the other words in the sentence that contribute to it having the same meaning overall. Did you downvote my answer just because you did not understand the explanation? – user21820 Mar 23 '16 at 16:11
  • @Gilles: Furthermore, it is very unkind of you if you did that since the question was clearly the general one about "on" versus "about" when referring to the topic. The question itself says "for example". – user21820 Mar 23 '16 at 16:13

The preposition "about" generally denotes some kind of circumscribing. That is why you can walk about a place, or talk about something (circumscribing the topic using words). This also explains some idioms like "beat about the bush" (instead of "going straight to the point"; note the variant "beat around the bush") and constructions like "how/what about ...?"

In contrast, the preposition "on" generally denotes some kind of position on some kind of surface, loosely interpreted. That is why you say "on the earth" and "on the skin". When we say "conversation on X" we are conveying that the conversation was based on X (as if X was the ground on which the conversation proceeded). This is also why we say "on-topic" and "off-topic" and "on the other hand". The default meaning of "on" also makes it wrong to use "on" in certain cases, which may even differ between verbs for speaking.

Consider the following:

  1. said something on eating habits (correct; note that "say" must have a direct object)

    said something on food (correct)

    said something on the plane (correct but "the plane" is not the topic here!)

  2. spoke on eating habits (correct)

    spoke on food (somewhat weird)

    spoke on the plane ("the plane" is not the topic)

  3. talked on eating habits (somewhat weird; note that "a talk on eating habits" is fine)

    talked on food (somewhat weird; note that "a talk on food" is fine)

    talked on the plane ("the plane" is not the topic)

  4. asked on eating habits (WRONG)

    asked on food (WRONG)

    asked on the plane ("the plane" is not the topic)

  5. lied on eating habits (WRONG)

    lied on food (WRONG)

    lied on the plane ("the plane" is not the topic)

Similarly for nouns derived from such verbs. For example a "conversation on the plane" is not the same as "a conversation about the plane".

In contrast, all the verbs and nouns for speaking can be used with "about" to denote the topic, but of course one has to fulfill the grammatical requirements of each verb:

  1. said/mumbled/shouted something about food

    spoke/talked/asked/lied/conversed about food

  2. a speech/talk/question/lie/conversation about food

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