View source for Help:Introduction to Categories

'''Categories''' are a way to tag articles in a way which makes them easy to find. A category is just a collection of articles, and other categories. The 'magic' which makes them useful as a way to find things is the way they are arranged.

Categories are arranged in a hierarchy, but it is not a strict tree; a category may be included in several different super-categories (e.g. [[:Category:IBM PCs]] is in both [[:Category:IBM Computers]] and [[:Category:Personal Computers]]). This is because the category system's primary goal is to help find things; having alternate 'paths' to things makes them easier to find. (So for someone interested in IBM gear, they might get to [[:Category:IBM PCs]] via [[:Category:IBM Computers]]; but some who is interested in PCs, they might get to it via [[:Category:Personal Computers]].) 

To see a list of existing categories, click [[Special:Categories|here]]. (That only shows categories which have at least one page in them; for a complete list of all categories, including empty ones, click [[Special:Allpages/Category:|here]].)

==Syntax==

To add an article to a category, simply put <tt><nowiki>[[Category: Category Name]]</nowiki></tt> in an article. Convention dictates that this be at the very bottom of the article, but in principle it could be anywhere. Convention on the CHWiki also is that all category names are fully capitalized.

This creates the category if it did not already exist. However, since the 'header page' (not the formal MediaWiki term for it, but I'm not sure what is) of a new category is empty of any descriptive text, it will still appear as a red link until there is text in the category's header page (which you can edit as you would a regular article).

To arrange a sub-category of a category, simply edit the sub-category's header page to place the sub-category in the parent category, just as you would to place an article in a category; the sub-category will then automatically be placed in the parent category.

To link to a category, without placing the page which holds the link in that category, prepend a colon to the link, like this: <tt><nowiki>[[:Category: Category Name]]</nowiki></tt>.

==Organization==

To avoid needing to tag an article with many categories, we are attempting to organize the categories to mostly be in a hierarchy, and only tagging articles with the most restricted category. That will automatically make them available through the higher-level categories as well.

For instance, the category [[:Category: Memories]] contains the sub-category [[:Category: DEC Memories]], which then contains the sub-sub-categories [[:Category: UNIBUS Memories]], [[:Category: QBUS Memories]], etc. Tagging an article with just one of the latter automatically enables it to be found via any of the more encompassing categories.

Sub-categories may also appear in more than one super-category. This allows all the articles in a sub-category to only need a single category tag, to be reachable via either super-category.

It is possible to tag an article too with more than one category, which is occasionally appropriate; see the [[operating system]]s for [[Digital Equipment Corporation|DEC]] machines example below.

Some articles may need to be placed higher in the hierarchy than other seemingly-similar ones, if there is no appropriate sub-category; e.g. [[Multics]] has to go in the top-level category [[:Category: Operating Systems]] since it was ''sui generis''.

If you go to tag an article, and you can't find a logical category for it - that might be because there ''isn't one yet''. Don't just cram the article into some category that isn't a good home for it; that is damaging the main purpose of the category system on the CHWiki - which is to make it easy to find things. If an article is put into a category that isn't a clearly obvious one for it, that will just make it harder to find it in the future. So, if it seems like the 'right' category is missing - ''please'' drop us a note, [[Help talk:Categories|here]], so we can think about what the right one should be.

===Current organization===

Here are a few details about the current category organization for the more filled-out areas of the Wiki, e.g. DEC computers (which is used here as an example solely because it is currently the most complete).

At the top are a set of 'meta-categories', all included in an overall category, [[:Category: Digital Equipment Corporation]] (which is itself in the top-level category [[:Category: Manufacturers]]): [[:Category: DEC Architectures]], [[:Category: DEC Buses]], [[:Category: DEC Memories]], [[:Category: DEC Processors]], [[:Category: DEC Systems]], etc. (These are themselves generally also included in matching wiki-wide 'super-categories', e.g. [[:Category: Architectures]], [[:Category: Buses]], etc.)

Beneath this are categories, similarly divided, for each line: e.g. [[:Category: PDP-10 Processors]], [[:Category: PDP-11 Processors]], etc; these are included in the appropriate DEC-wide category. (The difference between the 'Systems' categories and the 'Processor' categories is that the latter applies only to the [[Central Processing Unit|CPUs]], whereas the former includes complete systems, which include [[main memory]], [[disk]]s, etc.)

There is also a category for each product line, which includes everything to do with that line; it includes the per-line categories for that line, and also articles about the line which don't fit any of those categories. See, for example, [[:Category: PDP-11s]].

Finally, in some cases, there is additional complexity.

E.g. since there are many OSs for DEC machines, but which were not written by DEC, there is a further category for these 'non-DEC OSs': [[:Category: Non-DEC Operating Systems]]. (This is in addition to [[:Category: DEC Operating Systems]], for those written by DEC; and per-line categories - e.g. [[:Category: PDP-8 Operating Systems]], [[:Category: PDP-10 Operating Systems]], [[:Category: PDP-11 Operating Systems]], etc.) This category allows all these 'non-DEC operating systems for DEC machines' to be found easily. Articles on OSs for DEC machines will thus be tagged with two categories; one for the line, and one for their DEC/non-DEC origins.

There is no fixed answer as to whether it is better to create a sub-category which can be included in two super-categories (as with the IBM mainframe example), or instead include articles directly in two sub-categories (as with the OSs for DEC machines). The 'two-tags' route was taken in latter case to prevent the creation of too many very small categories. (See the discussion about this case [[Help talk:Categories#Other cats|here]] for guidance for similar cases.)

==See also==

* [[Help:Categories]] - discusses the overall structure of the categories on the CHWiki

==External links==

* [https://www.mediawiki.org/wiki/Help:Categories Help:Categories] - documentation on categories at MediaWiki

[[Category: Help]]