From what I understood about tag pruning, tags which are used only on one question are automatically removed if they are used only on one question. This happens after the tag is 6 months old (if it did not attract other questions in that period). The exception is that tags which have a tag-wiki are not removed even if there is only one question.

I have noticed the tag (mereology) on MathOverflow. This tag was created in March 2018. When I tried using SEDE, I did not find any other questions which had this tag at some point. (Questions where this tag was removed, All questions which had this tag at some point, Deleted questions with this tag.) Of course, it is still possible that there are some deleted questions with this tag. (Since for deleted questions I cannot get revision history from SEDE - only the tags which the question had at the time of deletion.) The tag-info for this tag is empty. I have noticed that the tag-creator suggested a tag-excerpt, but the edit was rejected.

Are there some conditions for the tag pruning script which I have missed?

TL:DR; Is there some reason why this tag wasn't removed, despite being much older than 6 months and having empty tag-wiki?

I will add that I am posting this just because I want to understand a bit better how tag pruning actually works. Whether or not this specific tag might be useful and should be kept is up to the decision of the users of that specific site.

EDIT: I have noticed in the Wayback Machine that previously the tag existed on another question. Here are snapshots of lists of questions with this tag from July 2017 and from March 2019. However, for the question which had this tag in 2017, there is no trace of this tag in the revision history - for which the most likely explanation is that it has been either burninated or removed by the tag-pruning script. Could possibly the fact that the tag with the same name has already been removed by the same script be the reason why it was not applied in this case?

  • 2
    Sites can turn off the automatic tag pruning. We've done so on Arqade, due to the penchant for single questions for rarely played games.
    – fbueckert
    Commented Mar 12, 2019 at 14:10
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    Interesting, I didn't know that. But recently some tags have been pruned on MathOverflow, e.g. here.
    – Glorfindel Mod
    Commented Mar 12, 2019 at 14:13
  • @fbuecker Thanks, that is interesting to know. However, this does not seem to be the case for this site - I have seen several tags being removed after the time which corresponds to the "single question in 6 months rule"; such as (wittgenstein) or (vinogradov-bracket).
    – Martin
    Commented Mar 12, 2019 at 14:13
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    Maybe the suggestion here was implemented at some point. Commented Mar 12, 2019 at 14:21
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    @fbueckert Latest update is that SE refuses to make such exceptions any more. Commented Mar 12, 2019 at 15:06

1 Answer 1


So this is interesting. Technically speaking, that tag does have a wiki. It's just blank. The wiki's post ID is assigned to the tag, and you can view the revision history showing it was created shortly after the post was created: https://mathoverflow.net/posts/295234/revisions

Having the wiki created but never getting content can only ever occur when there is a suggested edit for the tag wiki or excerpt and it was rejected. The system creates the wiki immediately so that the suggested edit has something to be attached.

Since the tag does "have" a wiki, it is not eligible for auto-pruning.


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