XML in a Nutshell, 3rd Edition

Selecting the Parent Element with ..

A double period (..) indicates the parent of the current node. For example, the XPath expression //@id identifies all id attributes in the document. Therefore, //@id/.. identifies all elements in the document that have id attributes. The XPath expression //middle_initial/../first_name identifies all first_name elements that are siblings of middle_initial elements in the document. Applied to Example 9-1, this selects <first_name>Richard</first_name> but not <first_name>Alan</first_name>.