Wikipedia:Wikipedia Signpost/2008-03-03/Hidden Categories
As of February 20, 2008, in revision 31113, categories with the magic word __HIDDENCAT__ on them will not show up as a category on their member pages (see archived story). Originally, this caused parent categories to also not show up on their sub-categories' pages, but that was fixed in revision 31121. There is also an option in a user's miscellaneous preferences to show all hidden categories. Currently, the best way to hide a category is to place {{hiddencat}} (which is actually a template, unlike {{DEFAULTSORT}}, which is a magic word that includes the braces) on it, which adds not only the magic word, but also a notice informing readers that the category is not shown.
There is currently much discussion over which categories the magic word should be placed on. The current consensus seems to be that any category that categorizes the article should be hidden, whereas categories that categorize the subject need to be shown. For example, Category:Articles that need to be wikified is hidden, but Category:Percussion instruments stays visible. Category:Hidden categories (which is not, for the record, a hidden category) lists all currently hidden categories. However, consensus is not clear about whether or not stub categories need to be hidden. Some argue that they are maintenance categories and do not directly contribute to the encyclopedia, while others argue that hiding stub categories would severely hinder the processes of stub sorting and categorizing. Also, stubs tend to have few categories, and sometimes only a stub category categorizes an article.
Some users have also suggested using hidden categories as a way to crudely implement category intersections. This would basically work by adding an article to all categories that applied to it, and hiding the sub-categories. The example given in the discussion, the George Washington Bridge, would be visibly in the categories for bridges, things completed in 1931, things related to the Hudson River, suspension bridges, toll roads, etc. It would also be in categories such as bridges completed in 1931, toll suspension bridges, and toll bridges completed in 1931, but these would be hidden. Some users were concerned about the sheer number of new categories this would entail, though. In the example, the George Washington bridge would also be categorized under toll suspension bridges completed in 1931 related to the Hudson River. Each extra primary category on a page would add more and more sub-categories.
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