What Tags are

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This page intends to give a brief introduction into the concept of tags in OpenStreetMap and what is important to know about them in general.

Tags in OpenStreetMap are free form attributes the features in the OpenStreetMap database can have assigned. These tags have the form of key=value pairs. Free form here means mappers can freely choose what key=value pairs they assign to features. And this freedom is not just theoretical, mappers in OpenStreetMap are actually encouraged to invent new tags on the spot whenever there is a need to characterize something for which so far no tagging has been established.

The only major constraint is that a single feature cannot have more than one tag with the same key. Some derive from this purely technical constraint that different tags with the same keys are mutually exclusive or by definition represent non-overlapping classifications. That however is not the case. With very few exceptions it is tags (meaning key=value combinations) that carry meaning in OpenStreetMap and keys alone do not. Therefore TagDoc only documents tags and not keys - with the exception of the few keys carrying meaning on their own - see Tag Categorization.

Although mappers are free in assigning tags to features they map without there being any authoritative rules to follow, this only results in meaningful information if there is some agreement on the meaning of those tags. This agreement traditionally forms through the informal (and mostly non-lingual) communication between mappers using the tags and mappers observing other mappers in using the tags. In addition mappers are encouraged to document their use of tags on the OpenStreetMap Wiki and thereby verbally communicate about and discuss the meaning of tags - which however comes with the inevitable cultural biases and social exclusivity of verbal communication. And finally data users like the producers of maps as well as the developers of OpenStreetMap editors and of the tagging presets in those also influence the building of this agreement on the meaning of tags in a significant way.

By documenting the de facto use of tags in OpenStreetMap, TagDoc tries to put down and communicate whatever agreement there is on the meaning of tags straight from the horse's mouth so to speak. See TagDoc:About for more details on the basic premises of the project.