<p>Tags on SE can be made synonyms with or without merging.</p>

<p>Non-merged synonyms have a bit of a weird behaviour. Take a look e.g. at <a href="https://stats.stackexchange.com/questions/tagged/books" class="post-tag" title="show questions tagged &#39;books&#39;" rel="tag">books</a> which is a synonym of <a href="https://stats.stackexchange.com/questions/tagged/references" class="post-tag" title="show questions tagged &#39;references&#39;" rel="tag">references</a>. If I search for [books], I get redirected to the search for [references], and the questions tagged with [books] are included into the search, try <a href="https://stats.stackexchange.com/questions/tagged/books">https://stats.stackexchange.com/questions/tagged/books</a> (880 results). If somebody writes a question and tags it with [books], it will automatically be re-tagged to [references] at the moment of posting. If somebody edits an old question tagged with [books], it will automatically be re-tagged to [references] too. If I see a question tagged with [books] and mouse-over this tag, I see tag wiki excerpt from [references].</p>

<p>However,</p>

<ol>
<li>questions tagged with [books] are still displayed with [books], and </li>
<li>if I browse <a href="https://stats.stackexchange.com/tags">https://stats.stackexchange.com/tags</a>, I see [references] with only <code>× 713</code> question count and [books] appears with <code>× 168</code> question count and its own tag wiki excerpt (which can be seen pretty much nowhere else) --</li>
</ol>

<p><strong>and both of these things are somewhat annoying.</strong></p>

<p>"Merging" simply means that all the questions with [books] are silently and instantly re-tagged to [references]. According to the meta.SE thread <a href="https://meta.stackexchange.com/questions/70710/what-are-tag-synonyms-and-merged-tags-how-do-they-work">What are tag synonyms and merged tags? How do they work?</a> (emphasis mine),</p>

<blockquote>
  <p><strong>All good tag synonyms should eventually be merged (<a href="https://meta.stackexchange.com/questions/262280/show-total-number-of-questions-in-synonym-group-at-all-occasions/262288#262288">source</a>).</strong> This retags all questions tagged with a synonym – which is not automatically done when a synonym is created. As merging is not (easily) reversible, it should be done with caution and only when a synonym is established. Only moderators can perform a merge.</p>
</blockquote>

<p><strong>So, shall we be merging all our "established" tag synonyms?</strong> How should we define when a synonym is "established" enough? I would say 1-3 months trial period after a synonym is created should be enough. If so, should we go over all existing synonyms and do the merges?</p>

<p>There is even a special tab for the un-merged ones on the tag synonyms page, clearly indicating that the SE system is inviting the mods to do the merges:</p>

<ul>
<li><a href="https://stats.stackexchange.com/tags/synonyms?filter=merge">https://stats.stackexchange.com/tags/synonyms?filter=merge</a></li>
<li><a href="http://stats.meta.stackexchange.com/tags/synonyms?filter=merge">http://stats.meta.stackexchange.com/tags/synonyms?filter=merge</a></li>
</ul>
