The axis portion of an XPath location step specifies, like the axis of a two-dimensional graph, the direction in which to locate something relative to some starting point. There is an axis corresponding to each of the basic XPath tree model relationships:
parent::child::ancestor::descendant::ancestor-or-self::descendant-or-self::preceding::following::preceding-sibling::following-sibling::self::attribute::namespace::Use XPath Axes to Navigate through XML Data