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.

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designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.
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You need help with the use of one or more of the site's features.
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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…
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For discussions and questions on the appropriateness (for the main site) of a question or a certain type of question.
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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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The process of migrating off-topic questions from one Stack Exchange site to another.
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Questions related to the closely related website, https://math.stackexchange.com
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Questions and discussions on and related to the reputation point system used on the main site.
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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 related to closed questions (and on-hold question) and also process of closing in general.
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for questions deleted from the site. The tag 'specific-question' should also be used when referencing a specific question that has been deleted.
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Questions dealing with moderator actions, pages, abilities or decisions.
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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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Questions specifically about the process of asking questions on this site.
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Questions about sizes and other quantities of MathOverflow, e.g., number of questions, GB/day transferred, voting statistics, etc.
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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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For questions concerning citation of posts from MO and citation of other resources when posting here.
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Questions about diamond (♦) moderators, their powers, and actions they have taken.