An Essay on Derpibooru Tag System
in addition to the official Tag Guidelines
(based on the source code and field research)

Tags and Tag UI

Within the scope of Derpibooru,

Tag is a generalized text label that can be attached to (or removed from) an image, and describes a certain aspect of that image. Tags, attached to an image, are listed below the image on the view page. You can search for images that are, or are not tagged with a specific tag(s), and thus to find exactly (well, ideally) images you want to see, excluding images with content you prefer to leave unseen.

Tags, attached to an image (displayed under the image on its view page), or involved in a search query (displayed under the search bar), appear as colored "plates". A tag plate contains the tag's name in the middle, on the right is the number of images tagged with this tag, and on the left is one or several symbols: "+" for an unwatched tag, "-" for a watched tag, "S" for a spoilered tag, and "H" for a hidden tag.

When logged-in, and hovering mouse over a tag plate, a drop-down menu will appear that allows you watch, spoiler, or hide the tag respectively; the latter two — only if you use a custom filter, otherwise there will be a single line "Filter", bringing you to the Filters page, where you can choose a standard filter, or create a custom one.

Images with tags you watch will appear in your Watched feed, those with spoilered tags will be covered with a "Tag Hidden" overlay unless explicitly searching for that tags (to see the image anyway, hover over its thumbnail, or click if on its view page), those with hidden tags will stop exist for you even if explicitly searching for that tags. You can centralizedly manage the tags you watch in your Settings ("Content Settings"), and spoilered/hidden tags — in your Filters.

Categories and Sorting

Derpibooru tags are divided into several categories, recognizable by their unique colors. When displayed under an image, tags belonging to the same category are grouped together, and sorted alphabetically within their group. In turn, the sorting order for the groups (categories) is defined in Derpibooru's source code, as follows (the links lead to the tag search page with an example query for a corresponding category):

- Error (tags: "artist needed", "photographer needed", "source needed", "dead source", "useless source url");
- Rating ("safe", "suggestive", "questionable", "explicit", "semi-grimdark", "grimdark", "grotesque");
- Origin ("artist:…", "colorist: …", "editor: …", "photographer: …", "… edit", "… screencap", and such);
- Character (canonical character name tags);
- OC ("oc", "oc only", "oc: …");
- Species ("pony", "… pony", "pegasus", "unicorn", "human", and such);
- Body Type ("anthro", "unguligrade anthro", "plantigrade anthro", "semi-anthro", "digitigrade anthro", "taur");
- Fanmade Content (titles of fan-made (non-canonical) stories, comics, universes, blogs, events, and such; interestingly, "princess molestia" lies in this category, but there must be a perfectly reasonable explanation for that);
- Official Content (the same as for Fanmade Content, but for official (canonical) titles);
- Spoiler ("leak" and "spoiler: …" tags, mostly for seasons/episodes/issues you have not seen and don't want to spoil);
- Uncategorized (all other tags).

An interesting fact: in the source code, tags in namespaces ("prefixed with") "artist:", "colorist:", "editor:", "oc:", "photographer:", are treated as "underscore safe"; supposedly, underscores in other tags are replaced with whitespaces.

Another two beasts you may encounter on Derpibooru, are

Tag Aliases and Implied Tags

Sometimes, more than one tag can describe the same thing. It may involve synonyms, contracted forms, different word order, common misspellings, and so on. When such a situation arises, a good solution is to merge the tags. One tag becomes canonical; the rest (aliases), when occurs in a search query, or is added to an image, is automatically replaced with the canonical tag. Thus, people can use contractions to speed up their image search, or still get results while searching for a mistyped tag or a rare spelling.

Also, when tagging an image, it often happens that adding a certain tag necessitates adding one or more other tags that describe some characteristic aspects of the first tag (like, when adding the "humanized" tag, you want the image to get "human" as well). To automate it, a tag can be assigned with "implied tags", which will be added automatically once a user tags an image with the master tag.

Note: there is a discussion about including this info into the standard help. Feel free to vote and comment!