Due to several discussions on the main site, I am asking this question concerning the best practices for tagging questions on MathOverflow. Unfortunately, the relevant documentation page https://mathoverflow.net/help/tagging does not answer the following questions (except partially questions number 2 and 4 below). Moreover, that help page is not specific to MathOverflow.

Ideally, the result of this discussion would either be added later to the documentation, or it would be turned into a FAQ thread. I hope I am not overstepping my bounds here.

  1. How should I tag a question on MathOverflow? Which tags are most helpful? Which tags should I not use?

  2. What makes a good tag?

  3. What are top level tags, and why should I try to use them? [It would be a good idea to list all the top level tags here, and perhaps their meanings as well.]

  4. What is a meta tag, and why should I not use them?

  5. Why are some common tags deprecated?

  6. What are the advantages of proper tagging?

Please feel free to add new questions, or to modify the ones above.

I hope this thread eventually leads to a sufficient consensus, and that its content can be a group effort. If the above are too many questions, perhaps we can separate them into several meta threads.


3 Answers 3


Here are some comments to start off on question 3. Feel free to edit.

What are top level tags, and why should I try to use them?

The top level tags are tags like and which begin with a two letter code. They roughly follow their arXiv counterparts, from which they originated.

I believe it is recommended to always use top level tags whenever possible. Most importantly, top level tags (and a few others like ) provide the highest level subject classification at MathOverflow. Also, they are more stable than most other tags, and their meaning also tends to be somewhat well established. Ideally, in the long term, this would help in archiving and finding MathOverflow questions.

Here is a list of all top level tags:


How should I tag a question on MathOverflow? (first steps)

Each (mathematical) question should ideally have among its tags, one can use up to five tags, (at least) one top level tag (for details on top level tags see the answer dedicated to top level tags).

Thus, a first step in tagging your question could be to choose a tag from the list of top level tags given in the just mentioned answer. (If you feel there really is no fitting top level tag for your question, at least try to use one of the more popular tags.)

In addition to this top level tag you then can add more specific (mathematical) tags to narrow down to a sub-domain of the top level subject or add additional top level tags for question touching upon different fields.

There exist rather specialised tags on MO, if you use one of them it can make sense to also use existing 'intermediate' tags, so better tag , , instead of only , (and please do not tag only ).

But, why can't I just only use when everybody who might answer knows this is about analytic number theory anyway?

A main point in using popular tags, such as top level tags, is that some users filter and browse the questions 'by tags' (and thus see only those question using certain tags), so some number theorists might never even see your question if you do not tag it .

  • $\begingroup$ I do not consider this as complete in any way, and might add to it later. $\endgroup$
    – user9072
    Commented Oct 20, 2013 at 15:06
  • $\begingroup$ Well -- isn't all this more or less self-evident for someone who cares enough to look what tags are there and how other questions are tagged? -- And will someone who doesn't care read such advice at all? -- I'm not sure. $\endgroup$
    – Stefan Kohl Mod
    Commented Oct 20, 2013 at 15:50
  • 2
    $\begingroup$ If things are written up one can link to them to inform new users (else one has to spell out a brief version each time). They then might or might not follow the advice. But, the point that some people filter and browse by tags and thus a special tag even if uniquely implying the 'higher' one is not sufficient might not be so self-evident. And, indeed, I got into some discussions with frequent users that do not seem to appreciate the structuring function of tags. $\endgroup$
    – user9072
    Commented Oct 20, 2013 at 16:09
  • $\begingroup$ I see. -- Indeed there is a notable difference between MO tags and the MSC -- among the former there is no hierarchy. Maybe this difference is worth to be mentioned explictly. $\endgroup$
    – Stefan Kohl Mod
    Commented Oct 20, 2013 at 16:19
  • $\begingroup$ Now I am not sure we mean the same thing but assuming we do: yes, a main point I meant to convey is that one should keep in mind that on MO giving something corresponding to a 5-character MSC does not in a technical sense put this in the relevant 3-char and 2-char category, so that one has to do this manually since also this technical sense has some relevance. $\endgroup$
    – user9072
    Commented Oct 20, 2013 at 16:28
  • $\begingroup$ Yes, that's what I meant. $\endgroup$
    – Stefan Kohl Mod
    Commented Oct 20, 2013 at 16:48
  • $\begingroup$ Some advice on choosing tags posted on Mathematics Meta: How am I supposed to use tags? (It is a different site - but some of the things mentioned there apply for MathOverflow, too.) $\endgroup$ Commented Dec 4, 2022 at 8:36

What are the advantages of proper tagging?

The purpose of tagging is (at least) two-fold.

One purpose is quite obvious. The tags are shown prominently below the question, when one looks at a lists of questions such as on the front-page, and thus can be used convey something about its content to allow users to decide whether to have a look at it.

Another purpose is to use tags, so to speak, to create lists of questions and more generally to give some structure to the ever growing number of questions on the site. A non-exhaustive list of things that can be done:

  • clicking on a tag, will create a list of all question using the tag.
  • searches can be conveniently restricted along tags.
  • one can subscribe to tags to receive email notifications of new questions using it or have the same functionality via RSS.
  • in ones preferences one can favorite (and ignore) some tags to change the appearance of questions tagged with them.

Thus, using appropriate tags one can increase ones chances that users competent to answer the question, or just interested in it, will notice the question in the first place. Conversely, using only very specialized tags (which likely almost nobody specifically favorited, subscribed to, etc) or worse just newly created tags, one might miss a chance to give visibility to ones question.


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