Hierarchy Diagrams

These diagrams illustrate portions of the hierarchical (nested) structure of this Tag Set. Each diagram has one element “root” and may illustrate the structures of several additional elements. For each element illustrated, its name appears in a box on the left, with lines to boxes for each of its possible child-elements, which are in a column on the right.

Boxes for child elements can have names and/or symbols within. If the box has merely “...”, it means that the actual content has been omitted, to save space or improve clarity. If it has merely an icon of a page with lines, it stands for text content rather than an actual element.

If a box has an element name, then symbols at the left end of the box indicate whether that element is required and/or repeatable. These symbols are called “occurrence indicators”:

?	item is optional (zero or one)
*	item may occur any number of times (zero or more)
+	item must occur at least once, but may occur any number of times (one or more)
	vertical bar on the left of the box: item is the “document element”: the top-most element, such as <article> or <book> and is therefore required
(no symbol)	item must occur exactly once

The symbols at the right end of a box have these meanings:

˜ (a tilde)	the item may take one or more attributes
	vertical bar on the right of the box: the item is expanded elsewhere (for example, if an element is permitted in more than one place in an element, it is expanded only once.)

The lines that connect a box to boxes on its right may either be squared-off or direct (angled) lines.

Squared-off lines mean the items to the right must occur in the order shown,
Angled lines indicate that any order is permitted.

	Required
	Optional. May occur zero or one times.
	Required, repeatable. May occur one or more times.
	Optional, repeatable. May occur zero or more times.

	Element content is expanded elsewhere in the diagram.
	The element has attributes.
	One or more elements collapsed for clarity
	Element is the root element of the model.
	Text, numbers, and special characters

	Required sequence: Element1 followed by Element2 followed by Element3
	Choice of sequence: Element1 or Element2 or Element3

	Zero or more repetitions of Element1 or Element2 or Element3 followed by zero or one occurrences of Element4
	Zero or one occurrences of either Element1 followed by Element2 followed by Element3 or one or more occurrences of Element4
	Either one or more repetitions of Element1 followed by Element2 or one or more occurrences of Element4