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In Brief

Hierarchical Trees in XML
Table of Contents
1 Internet Explorer's Structural View
2 Nodes
3 Trees in Linguistics
4 Related Pages

1 Internet Explorer's Structural View

Internet Explorer's default view for an XML file displays the document's hierarchical structure. The minus and plus signs allow the document to be displayed as a collapsible outline -- indeed, as a collapsible tree:

To view an XML document as a collapsible tree in Internet Explorer, make sure the document does not contain a reference to an XSL stylesheet. (In the image above, on the second and third lines, you can see that the stylesheet reference is commented out.)

In XML, various concepts, most of which spring from the way we speak about trees or families, are used to express relationships within a document's hierarchical tree structure. At the base of its hierarchy, each tree has a root element, and it can be seen in Internet Explorer by clicking on the first minus sign shown in the document. From the root node stems a hierarchy of other branches and leaves. Leaves are terminal elements: They do not contain child elements.

2 Nodes

Each element in a tree structure is called a node. Relationships among nodes are expressed using metaphors borrowed from families: The root node, for instance, is the parent of all the other nodes, called children; together they enter into a parent-child relationship. Although a parent element can have multiple children, each child node has exactly one parent node. XML uses such constructs as parent, child, sibling, ancestor, and descendant in keywords, expressions, and functions.

3 Trees in Linguistics

Hierarchical trees are also frequently used to express relationships in linguistics. The following example, marked up in XML, reveals the hierarchical structure of the atomic syntactic elements that make up a sample sentence:

<s>
  <np><det>The</det><n>woman</n></np>   
  <vp><v>saw</v> 
    <np><det>the</det><n>man</n>
      <pp><p>in</p>
        <np><det>the</det><n>park</n></np>
      </pp>
    </np>
  </vp>
</s>

More on Hierarchical Trees in Phonology.