Once a private character has been declared using a <glyph-data> element, the character need not be defined with the same <glyph-data> element every time it is used. The character can be reused by using the <glyph-ref> element to point to the full <glyph-data> element. The pointing uses the ID/IDREF linking mechanism, with the @glyph-data attribute of the <glyph-ref> element pointing to the @id attribute of an existing <glyph-data> element. Since the reference uses the ID/IDREF, the reference may precede or follow the element to which it is pointing.