The syntax for relative URLs is a shortened form of that for absolute URLs , where some prefix of the URL is missing and certain path components ("." and "..") have a special meaning when interpreting a relative path. Because a relative URL may appear in any context that could hold an absolute URL, systems that support relative URLs must be able to recognize them as part of the URL parsing process.
Although this document does not seek to define the overall URL syntax, some discussion of it is necessary in order to describe the parsing of relative URLs. In particular, base documents can only make use of relative URLs when their base URL fits within the generic-RL syntax described below. Although some URL schemes do not require this generic-RL syntax, it is assumed that any document which contains a relative reference does have a base URL that obeys the syntax. In other words, relative URLs cannot be used within documents that have unsuitable base URLs.