XML in a Nutshell, 3rd Edition

Axes

Each XPath location step moves along an axis from a context node. The context node is a particular root, element, attribute, comment, processing-instruction, namespace, or text node in the XML document. (In practice, it’s almost always an element node or the root node.) The context node may be a node selected by the previous location step in the location path, or it may be determined by extra-XPath rules, such as which node is matched by an xsl:template element.

However the context node is determined, it has some relationship to every other node in the document. The various axes divide the document into different overlapping sets, depending on their relationship to the context node. There are exactly 13 axes you can use in an XPath location step:

child

All children of the context node. Only root and element nodes have children. Attribute and namespace nodes are not children of any node, although they do have parent nodes.

descendant

All nodes contained inside the context node—that is, a child node, a child of a child node, a child of a child of a child node, and so on. Only root and element nodes have descendants. Like the child axis, the descendant axis never contains attribute or namespace nodes.

descendant-or-self

Any descendant of the context node or the context node itself. // is an abbreviation for /descendant-or-self::node( )/.

parent

The element or root node that immediately contains the context node. Only the root node does not have a parent node. .. is an abbreviation for ...