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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Questions about diamond (♦) moderators, their powers, and actions they have taken.
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For questions about single users having multiple accounts, known as sock accounts.
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For questions about MathOverflow newsletter - possibility of getting an email with top posts.
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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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an open standard that allows users to log in to many websites with a single set of third-party credentials.
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blocked from asking new questions, or posting new answers.
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for problems and suggestions for the privileges gained by reputation on the main site and on meta.
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Protected questions have the additional restriction that users with low reputation are not permitted to answer the question.
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The undisclosed algorithm through which all new questions are passed to determine if they can be posted on SE.
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Questions and discussions on and related to the reputation point system used on the main site.
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For questions related to how the site looks, feels and behaves when its responsive design comes into play
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For questions about the review queues.
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For questions about the various RSS feeds provided by Stack Exchange sites.
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for sandbox questions, which allow people to write answers meant for testing of a particular issue, or a draft for a later answer.
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For questions about built-in search functionality of this site and about searching this site using external search engines.
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the act of casting many successive upvotes or downvotes on the posts of a specific user.
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Questions related to the "side bar"; the bar that runs down the right side of the page on Stack Exchange sites.
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Questions about suspected user spam, the spam flag or combating spam as a community
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for questions on Meta that relate to a specific answer on MathOverflow. (If a linked answer serves only as an example of a more general issue you want to discuss, this tag should not be u…
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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 questions which focus on a particular user, rather than those discussing general user issues.
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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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Indicates that a feature request has been implemented, a bug has been fixed, or another type of request has been processed.