The <underline-start> and <underline-end> elements enable creation of underlines in content, most commonly in mathematical content, where a normal underline element (<underline>) would break the XML well-formedness rule that no two elements may overlap. The <underline-start> and <underline-end> elements act as milestone tags marking the start and end of underlining that may extend across element boundaries. The underline created using these two elements can have arbitrary start and end points because the start and end elements are paired through the use of the unique identifiers (IDs), not as the start and end of a single element. For example, in the text “ABCD”, if the text “ABC” is to be overlined and the text “CD” is to be underlined, ordinary underlining with <underline> would create invalid tag overlap. The text could be correctly tagged, using the <underline-start> and <underline-end> elements as:

...
<overline-start id="ov1"/>AB
<underline-start id="ul1"/>C
<overline-end rid="ov1"/>D
<underline-end rid="ul1"/>
...