On many SE sites it is common practice to delete comments that became "obosolete" and there is a flag reason for this. On a few other sites (especially MO) this is not done (or at least not much).

What should be the common practice regarding deletion of obsolete comments on MESE?

The direct motivation is that I just got such a flag on a comment. The comment was a reply to a (self-)deleted comment that suggested an edit, and the flagged comment announced an edit of the post that was done.

So it is obsolete. But also contained some piece of information that is now gone (yet which OP did not consider sufficiently relevant to include in the post in the edit while they could have).

I say "is" as I (tentativeley) deleted the comment following the flag (also since I observed OP self-deleting obsolete comments of their own).

Personally, I am fine doing this either way, but I think we should discuss it.

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  • $\begingroup$ Could you link to the example, or show it in your question? I'd like to get an idea of what such comments "look like". $\endgroup$ – Brendan W. Sullivan Apr 6 '14 at 17:27
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    $\begingroup$ @brendansullivan07 I think, he means comments like "You can please provide more details about the question?", then an edit happens - and the comment is then obsolote. $\endgroup$ – Markus Klein Apr 6 '14 at 18:16
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    $\begingroup$ @MarkusKlein: Aha I see. In that case, I say delete away, as long as it's clear that it's now obsolete. $\endgroup$ – Brendan W. Sullivan Apr 6 '14 at 18:21
  • $\begingroup$ @brendansullivan07 I added the example now in an answer. Sorry for the delay in responding, but since Markus Klein asnwered it basically, it felt not urgent anymore and I was a bit short on time the last days. $\endgroup$ – quid Apr 9 '14 at 19:26


I refer you to the commenting privilege page:

Comments are temporary "Post-It" notes left on a question or answer. They can be up-voted (but not down-voted) and flagged, but do not generate reputation. There's no revision history, and when they are deleted they're gone for good.

Comments aren't meant to stick around for longer than a few weeks. Ideally, all useful information would be in posts where it's supposed to be.

But that's not how it works. People put actual, valuable stuff in comments. This information can and should be moved to answers at some point, but that's not the point here.

The point is that people tend to do this:

You really should add that teaching your students using an upside-down schoolbus may cause them to dream about the Cold War. - aGuy

Added, thanks - postAuthorGuy

Comment exchanges like above are useless, redundant information and should be deleted. As a mod, you don't need to be running around hunting for these, but if it bugs someone enough that they flag it you can and should delete it.

And then there's the somewhat official policy on mSO:

The [obsolete comment threads] don't add anything other than a distraction. I am often filled with disappointment when I see what looks like an interesting comment thread only to find it's mainly bunch of correction please and thank you's. I wouldn't mind seeing mine disappear.

And as yet another mSO answer says:

That's exactly what the obsolete flag is for. Flag away!

Comments are, by their very nature, transitional. Flagging helps keep them under control and tidied up.

So please delete them when they're obviously obsolete and someone flags them. Please.

If they contain more information than what's found in the post, and it's a similar comment exchange, edit it into the post! Then you can delete the comments since they're no longer needed.


The answer by Undo is pretty close to my own thinking; if there are no objections we could proceed along these lines.

To answer a request, here is the actual example. It was a comment on https://matheducators.stackexchange.com/a/1075/ by OP saying

@msh210 You are right: I checked through my first text on group theory (Abstract Algebra by Dan Saracino - a wonderful book) and, indeed, both Gauss' Lemma and The Eisenstein Criterion are covered (on pages 194 and 195, respectively). Will edit accordingly!

This was in reply to a comment saying.

+1; good point; many thanks. Just a side point: Re "one that I did not see until a course on Galois Theory", Eisenstein's criterion is mentioned in Herrstein's blue book Topics in Algebra, which is a book for a first course in abstract algebra. (I'm guessing it's also in other, similar books, but don't have any handy to check.)

Yet this comment was already deleted, when I got the flag.

A reason in favor if deletion here is that the conversation is incomplete (one being deleted) and this is also tricky to notice for the "other side" in the conversation as there is not notification for this. A reason against deletion is that the comment contains more specific information than what was included in the post. Thus, I had some doubts.

Mildly related note: there is another comment flag 'too chatty.' There I am quite hesitant to delete anything, since as opposed to 'obsolete' it would directly override some user's intent. For now I only intend to delete as 'too chatty' in very clear cases, and at least it should involve multiple comments (and based on exchanges we had I believe the other moderators will not delete isolated comments as 'too chatty' either [please correct me if this should be different].)

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  • $\begingroup$ For the "too chatty" thing. I think, comments only containing "Thank you" and similar should be deleted (or not just not be made!) $\endgroup$ – Markus Klein Apr 10 '14 at 6:28
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    $\begingroup$ @MarkusKlein I disagree strongly with that. I know this is the general "SE culture", but that does not mean it needs to be the norm on this specific site. Unless such comments start taking up a lot of space in the same place, I see no problem with them. $\endgroup$ – Tobias Kildetoft Apr 10 '14 at 7:19
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    $\begingroup$ For comments containing only 'thank you' a compromise could be to delete them after some time. Such things can be nice but also somehow become 'obsolete' once the recpient read it. I also would not keep a 'post it' saying 'thank you' that somebody put on some book I borrowed them and they returned in my absence, while at the moment it might have been a nice thing. $\endgroup$ – quid Apr 10 '14 at 9:22

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