The <overline-start> and <overline-end> elements act as milestone tags marking the start and end of overlining that may extend across element boundaries, mostly within mathematics, when ordinary MathML underline and overline functions are inadequate.

The <overline-start> and <overline-end> elements enable creation of overlines in content, most commonly in mathematical content, where a normal overline element (<overline>) would break the XML well-formedness rule that no two elements may overlap. The overline 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 overlining with the <overline> element would create invalid tag overlap. The text could be correctly tagged, using the <overline-start> and <overline-end> elements as:

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