A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.

designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.
You need help with the use of one or more of the site's features.
146 questions
For discussions related to some particular question posted (or to be posted) on the main site. Do not use if your intention is to discuss/ask about a general issue, and you merely linked some question…
For discussions and questions on the appropriateness (for the main site) of a question or a certain type of question.
Indicates that a feature request has been implemented, a bug has been fixed, or another type of request has been processed.
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Bug reports and suggestions for the MathJax system used to compile basic LaTeX code on the site.
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For questions about the design of MathOverflow. This includes bug reports and suggestions about the user interface.
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aimed for discussions about etiquette: how to behave on the site and the chat.
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The process of migrating off-topic questions from one Stack Exchange site to another.
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Questions and discussions on and related to the reputation point system used on the main site.
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Questions related to the closely related website, https://math.stackexchange.com
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For questions related to closed questions (and on-hold question) and also process of closing in general.
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a way to get additional attention for a question by offering some of your own reputation for great answers.
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for questions deleted from the site, or in a deleting process. The tag 'specific-question' should also be used when referencing a specific question.
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Questions specifically about the process of asking questions on this site.
Questions dealing with moderator actions, pages, abilities or decisions.
Questions about sizes and other quantities of MathOverflow, e.g., number of questions, GB/day transferred, voting statistics, etc.
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For questions about the flagging process, how flags are handled, etc. Flagging is the process of bringing a post to a community moderator's attention for any reason.
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the most frequently asked and answered questions on Meta.
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awarded to encourage and incentivize positive community actions within the site.
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A tag for questions about the community wiki feature (regardless of whether or not the questions themselves are community wiki).
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For questions concerning citation of posts from MO and citation of other resources when posting here.
18 questions