<sch:rule context="tei:rt/@from">
<sch:assert test="../@to">When from= is present, the to= attribute of <sch:name/> is required.</sch:assert>
</sch:rule>
to
points to the ending point of the span of text being glossed.
<sch:rule context="tei:rt/@to">
<sch:assert test="../@from">When to= is present, the from= attribute of <sch:name/> is required.</sch:assert>
</sch:rule>
Where the place attribute is not provided on the <rt> element, the default assumption is that the ruby gloss is above where the text is horizontal, and to the right of the text where it is vertical.
Example
The word 大統領daitōryō (president) is glossed character by character in hiragana to provide a pronunciation guide.