What are tag synonyms and merged tags? How do they work?

Question

• What are tag synonyms?
• How can tag synonyms be distinguished from "real" tags?
• How are new tag synonyms created?
• Who can create tag synonyms?
• How can I delete/reverse/undo bad tag synonyms?
• What are merged tags?

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Answer 1

There's a lot of good info in the announcement of tag synonyms on the Stack Overflow blog.

What are tag synonyms?

A tag synonym is usually a tag that has exactly the same meaning as some other tag, such as bike and bicycle. In some cases, tags that are subsets of other tags will also be considered synonyms, such as java-se for java.

The system organizes tags in a master–synonym relationship. All attempts to use the synonym tag for any given master tag are automatically converted to the master tag. So, users can enter a synonym tag when writing a question, but the master tag will be displayed when the question is loaded. Editing a question tagged with a synonym tag causes it to be replaced by the master tag. Similarly, when users search for questions tagged with a synonym, a list of questions tagged with the master will be displayed.

However, existing questions tagged with the synonym tag that were using it before the synonym was created and were not edited later will continue to use the synonym tag.

A master list of synonyms is available at /tags/synonyms.

How can tag synonyms be distinguished from "real" tags?

Tag synonyms are replaced by the respective master tag on almost every occasion, e.g., when you search for a tag, try to apply a tag to a question, or follow a link to the tag.

Note that it is rarely necessary for a user to make this distinction.

How are new tag synonyms created? Who can create tag synonyms?

• Users with the suggest tag synonyms privilege (2,500 reputation on sites with "full" requirements or 1,250 reputation on sites with "beta" requirements) and a total answer score of 5 or more on a given tag may suggest synonyms for that tag.
• Users with a net answer score (total upvotes minus downvotes) of 5 or more for a given tag may vote for synonyms for that tag, regardless of reputation.

Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2.

Additionally, moderators can create synonyms as needed on all tags and have their suggestions instantly approved.

Normal users can only propose or create synonyms for a given tag if the proposed synonym tag already exists. If a tag corresponding to a pre-existing synonym proposal ceases to exist (e.g., was removed from all questions, was merged with another tag, was burninated by an employee, etc.), those synonym proposals will be automatically deleted. Moderators are exempt from this restriction.

If you don't have enough reputation to create tag synonyms or you don't have enough answer score in the tag, make a post on your site's meta tagged discussion tags synonym-request explaining why you believe the synonym is necessary. (For synonym requests on this site, Meta Stack Exchange, or the per-site meta itself, add meta.) This is also a good idea if there aren't enough people aware of the proposed tag synonym, if you aren't sure whether the synonym is a good idea, or if there aren't enough users on the site who are able to vote for the synonym.

You can view synonym proposals for a specific tag at /tags/TAG_NAME/synonyms (e.g., https://stackoverflow.com/tags/jquery/synonyms). Additionally, a full list of all tag synonyms and synonym proposals is available at /tags/synonyms (e.g., https://meta.stackexchange.com/tags/synonyms).

Again, when a synonym is created, the synonymous tag is not replaced from existing questions that use it, but it cannot be added to new questions anymore (the master tag will be added instead). This is so that mistakes can easily be reverted.

How can I delete/reverse/undo bad tag synonyms?

Only moderators can correct bad synonyms. Therefore, the only recourse is to post a question on your per-site meta explaining the problem with the synonym relationship. Use the tags described above. Give the community a few days to show support or make alternate proposals. Once the discussion has settled, a moderator will perform the correction the community deems necessary. (If there is no moderator response a couple of days after clear community consensus is established, contact a moderator in chat or flag the meta post for moderator attention.)

As the original tag that was added as synonym is not deleted but is simply hidden through the redirect to the master tag, deleting the synonym will restore the original tag page including the tag wiki and all other information. Questions which have been asked or edited in the meantime will be tagged with the master tag though, so manual effort is required to retag those when a synonym is removed.

What are merged tags?

All good tag synonyms should eventually be merged (source). This retags all non-deleted questions tagged with another tag – which is not automatically done when a synonym is created. As merging is not (easily) reversible, it should be done with caution. Only moderators can perform a merge.

Merging, by itself, does not create a synonym, meaning that the tag that was merged can be recreated and used later. In most cases, you will want to create or ensure that a synonym exists, so that future attempts to use the merged tag will replace it with the canonical tag instead – moderators have the option to do so in the UI for merging tags. However, merging without a synonym is useful in certain cases, such as renaming a tag.

If you want to propose a tag merge request, open a discussion on your site's meta, tagged discussion tags retag-request. For questions about merging tags on this site, Meta Stack Exchange, or the per-site meta itself, add meta.

For more information, see this section of the moderator cheat sheet.