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It is well-known that creating a synonym and merging two tags are two completely independent actions1 - sometimes a synonym is created and the tags aren't merged, sometimes two tags are merged without creating a synonym between them.

The tag synonyms which aren't merged are quite common. What are the consequences for regular users? What do they have to keep in mind when using tags which are synonymized but not merged?

It is a bit unclear to me how actually the synonyms behave w.r.t. to searching. At the very least, I can tell from past experience that the results when searching for some phrase with [master-tag] and with [synonym] are not always the same. And sometimes the question does not appear neither when searching with the master tag nor when searching with the synonym.2

Without merging, various counts and scores are still calculated for the two tags separately. For example, this influences the list of top users and, consequently, whether a user gets a gold badge and the dupehammer. Moreover, the dupehammer in the master tag does no apply to questions tagged only with the synonym.3 

Are there some other things which affect some use cases for tags other than the above? Is some of the information stated above incorrect?

1See the FAQ post: What are tag synonyms and merged tags? How do they work? The answer posted here explains merging quite well, too: What is the difference between a synonym and merging a tag?

2See, for example, here: Tag synonyms and searching

3As mentioned here: How does tag synonym affect top users list?,  Which questions count for the purpose of tag badges and dupehammer for the tag synonyms? and Gold-badge duplicate hammers don't appear to check tag synonyms as far as I can tell; is this so, and should it be fixed if so?

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TL;DR: Effectively, tag synonyms cause the target tag to shadow the synonym. This only causes problems if the synonym exists. Merging knocks over a domino that deletes the source tag.

A tag merge is a one-time action that:

  • rewrites the tags of existing questions.
  • does not bump questions.

A tag synonym is an ongoing remapping that exists for convenience only, and:

  • redirects the synonym's tag pages to the target's corresponding tag page.
  • substitutes tags in the tag picker.
    • might(?) also perform this substitution server-side. I haven't checked.
  • substitutes tags in searches.
  • kinda assumes the synonym tag doesn't exist.

Tag merges are a destructive operation: since they don't bump anything and there's no undo button, they're not something you want to trust any old 2.5k user with. If synonyms automatically merged the tags, they would be really hard to correct if made in error; but synonyms without merges shouldn't really exist.

Since tag merges (under usual circumstances) cause a tag to be deleted, this will also delete tag badges.

Tag synonyms preserve tag wikis, but the redirecting behaviour makes it impossible (under normal circumstances) to access them. Tag merges remove all questions from the source tag, which will cause the tag to be deleted: the orphaned wikis are then visible to moderators.

In practice, an unmerged tag synonym will behave incorrectly. The software is not really designed for this case. Ask a moderator to merge the tags.

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  • I guess persuading moderators to merge the tags might not always be easy. For example, if I look at SO, there are almost 1000 such synonyms. (Admittedly, in many cases the smaller tag has less than 100 questions.) When running the same query on the site you moderate, there are only a few such synonyms.
    – Martin
    Commented Jun 17 at 9:55
  • @Martin It should be pretty easy: just post a meta question saying "hey can we fix this thing?", and they'll get around to it eventually. (It's just a matter of running down a list and clicking a button on each entry.)
    – wizzwizz4
    Commented Jun 17 at 14:55
  • Well, your experience with the mods seems to be different than mine. (Of course, this might depend on the site.) We'll see how it works out on this site and on Mathematics.
    – Martin
    Commented Jun 17 at 15:05
  • @Martin It might be worth linking the tool page for moderator convenience.
    – wizzwizz4
    Commented Jun 17 at 15:08

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