I am endlessly fascinated with content tagging systems
I worked with the Wikipedia category system a few years ago, and you could see the problems with hierarchical tagging systems right in action back then. (Though it may have gotten better in the meantime)<p>The system appeared simple: There were just two relations, &quot;Article A is a member of category B&quot; and &quot;Category X is a subcategory of category Y&quot;.<p>However, in practice, the community was using this system to represent a whole host of wildly different relationships between items, often with different implications what a category actually applied to.<p>E.g., if A has a subcategory B, this could mean one of several things: B might be an additional constraint on the items in A (&quot;American writers&quot; -&gt; &quot;19th century American writers&quot;), the <i>things</i> in B might be more specific than the things in A: (&quot;Writers&quot; -&gt; &quot;Novelists&quot;), A might apply to the <i>concept</i> B, not the things in B (&quot;Occupations&quot; -&gt; &quot;Writers&quot;) or A might refer to the <i>category</i> B (&quot;Categories with more than 100 entries&quot; -&gt; &quot;Writers&quot;) and on and on...<p>Of course those different aspects could even be combined. E.g. &quot;Categories with more than 100 entries&quot; might have a child &quot;Categories with more than 100 entries in need of review&quot;, which represents a constraint but might itself contain less than 100 entries...<p>The basic question &quot;Is item X in category Y&quot; becomes impossible to answer generally, because there is no clear indication if a category only applies to its direct children or to all of its descendants or only to the subcategories itself.<p>I&#x27;m sure there are sophisticated ontological systems which would allow users to specify all those different relationships separately. I&#x27;m also pretty sure that users would become sloppy after a short time or would disagree which particular relationship to use in a particular situation...