One use of tags is as a tool to add metadata to something, for example, a webpage or an instance of multimedia, such as an image. Metadata, defined as data about data, adds description to an item that is maintained separately from the item, but is linked to it. With a photograph, for example, metadata tags can identify the location where the photograph was taken, the photographer, the camera and lens used, the time of day, the identity of people and/or buildings in the photograph, etc. Metadata is also useful for locating and sorting items. A triple tag, also known as a machine tag, is a specialized type of tag that was first created in 2004 for Geolicious™ subsequently adopted by Mappr™ and GeoBloggers™, and was dubbed machine tag in January of 2007 when it was officially adopted by Flickr®.
A triple tag has a defined syntax and three elements, hence the name triple tag. The generic form looks like this:
Namespace:predicate=value
The namespace defines the realm of the tag. The predicate is a property of the namespace. And the value is a particular example of that property in that namespace.
One common use is to identify location, sometimes called geotagging. Geotagging looks like this, with the appropriate number substituted for the parenthesis:
geo:lat=(number)
geo:long=(number)
The number can be positive or negative, in which case a dash/minus sign precedes it.
Because the triple tag system is evolving as a folksonomy — in which individuals evolve tags as needed, as opposed to being instituted as a hierarchy, in which a standardized set of tags is set-up and shared for consistency — tags with the same meaning may use synonyms, rather than the same terms, while different taggers might also use the same term but with different meanings. The character definitions do seem to be consistent, however. The namespace and predicate, which are case-insensitive, must each begin with a letter a–z. The following characters can be a–z, 0–9, and underscores. Values are allowed to contain any value that a “plain vanilla” tag uses, but if spaces are included, the value must be wrapped in quotation marks.