On Mathematics Stack Exchange, there are several tag synonyms which haven't been merged; meaning that both the master tag and the synonym contain some questions.1

The situation when there is a synonym where the synonymized tag still contains some question is not optimal for several reasons.2 I'd consider the most important one the fact, that searching using tags does not return questions with the synonymized tag.3 (I will freely admit that for a long time I was fully unaware of this problem with synonyms which are non merged.)

At the same time, I understand that sometimes there might be good reasons to avoid merging or at the very least, not doing it immediately.

  • In general it's better not to be hasty with actions which are not reversible. (Once the tags are merged, we could only restore the original state by manual retagging.)
  • In particular if a synonym was created without discussion on meta, it is conceivable that some other users - who did not participate in creating the synonym - might be against it and might prefer to cancel the synonym and keep the two tags separate.
  • In some cases, the synonymized tags might need cleaning up; After the merging, we would no longer see which questions originally had the master tag and which had the synonymized tag.

If there are tags which are synonyms but haven't been merged, what is the right thing to do?

  • Should we just notify the moderators in some way? (And they will decide whether they merged the tags or whether some discussion
  • Is it better to make a post on meta and discuss it here? (And depending on the outcome of the discussion, the mods decide on further actions.)
  • Or should something else be done in cases like this?

This question was motivated by a particular tag synonym which was discussed recently in chat; but it is intended as a question about this situation in general, not about this specific pair of tags.

Here is another question on this meta related (to some extent) to the same issue and posted not too long ago: Tag synonyms: Inconsistencies.

1List of tags where both the master tag and the synonym contain some questions can be found using this SEDE query. In case you want to find the questions which still have the synonymized tag, you can find them using SEDE. (See also: Is there a way (e.g. with SEDE) to find questions with an existing tag that's been synonymized (but not merged) into another tag?) If you visit those questions, you can see that the synonymized tag is listed as one of the tags. (In fact, I posted those questions in the tagging chatroom - in the transcript, the current tags can be seen even if the tags are changed in the future.)

2See also: What are differences between synonyms with and without merging from the practical point of view?

3Here I mean searches that use a combination of tags and keywords. Let me try to explain this with an example. (Although I am fully aware that if the mods merge these tags, the example will no longer illustrate my points.) I will use this question: Relationship between ST-Path Ideals and ST-Cut Ideals? It is tagged , which is a synonym of . If you search for st path ideals, this question is among the search results if you use the tag graph-theory. But it is not among the results if you use graph-connectivity or st-connectivity. The question is still shown among the results if you search only using tags. Another interesting thing is different number of questions shown on the newest tab and on the active tab - apparently, only one of them includes the synonym.

Here is the list of the tag synonyms returned by the SEDE query mentioned above:

  • 1
    $\begingroup$ SO has almost 1000 tag synonyms that haven't been merged - so in comparison with that, the situation on this site isn't really that bad. $\endgroup$ Commented Jun 17 at 14:52
  • 1
    $\begingroup$ An example to expand on Martin's second paragraph above: once the tag 'propositional logic' (PL) is synonymised as 'first-order logic' (FOL) with no merger done, any onsite search for the tag 'propositional logic' no longer returns any result from set of existing posts that are tagged-PL-but-not-tagged-FOL. Thus, I feel that synonimising without merging tags is by design a transitory/interim/provisional state, and that (with consensus) either a full merger or a desynonymisation ought to be the end-point. $\endgroup$
    – ryang
    Commented Jun 20 at 11:18


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