The munderover element attaches accents or limits placed both over and under a base. It always sets displaystyle to "false" within underscript and overscript, but increments scriptlevel by 1 only when accentunder or accent, respectively, are "false". Within base, it always leaves both attributes unchanged. MathML Fundamentals
<math mathsize="80">
<mspace linebreak="newline" indentalign="center"></mspace>
<mtext style="border:1px;" mathsize="50">Underscript, Overscript munderover</mtext>
<mspace linebreak="newline" indentalign="center"></mspace>
<mtext style="border:1px;" mathsize="30">Left to Right</mtext>
<mspace linebreak="newline" indentalign="center"></mspace>
<mrow style="border:1px">
<mi>X</mi>
<munderover>
<mo>→</mo>
<mtext>maps to</mtext>
<mtext>maps to</mtext>
</munderover>
<mi>Y</mi>
</mrow>
<mrow style="border:1px">
<mi>X</mi>
<munderover overspace="-50%">
<mo>→</mo>
<mtext>maps to maps to maps to</mtext>
<mtext>maps to</mtext>
</munderover>
<mi>Y</mi>
</mrow>
<mspace linebreak="newline" indentalign="center"></mspace>
<mrow style="border:1px;">
<munderover style="border:1px">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<munderover style="border:1px" align="left">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<munderover style="border:1px" align="right">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<munderover style="border:1px" accentunder="true" accent="true">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<mstyle displaystyle='false' >
<munderover style="border:1px">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
</mstyle>
</mrow>
<mspace linebreak="newline" indentalign="center"></mspace>
<mrow style="border:1px;">
<munderover style="border:1px;">
<mrow style="border:1px">
<mi>x</mi>
<mo>+</mo>
<mi>y</mi>
<mo>+</mo>
<mi>z</mi>
</mrow>
<mo style="border:1px">⏟</mo>
<mo style="border:1px">⏞</mo>
</munderover>
<munderover underspace="-0.1em" overspace="-0.1em" style="border:1px;">
<mrow style="border:1px;">
<mi>x</mi>
<mo>+</mo>
<mi>y</mi>
<mo>+</mo>
<mi>z</mi>
</mrow>
<mo style="border:1px;">⏟</mo>
<mo style="border:1px;">⏞</mo>
</munderover>
</mrow>
<mspace linebreak="newline" indentalign="center"></mspace>
<mtext style="border:1px;" mathsize="30">Right to Left</mtext>
<mspace linebreak="newline" indentalign="center"></mspace>
<mrow dir="rtl">
<mrow style="border:1px">
<mi>X</mi>
<munderover>
<mo>→</mo>
<mtext>maps to</mtext>
<mtext>maps to</mtext>
</munderover>
<mi>Y</mi>
</mrow>
<mrow style="border:1px">
<mi>X</mi>
<munderover overspace="-50%">
<mo>→</mo>
<mtext>maps to maps to maps to</mtext>
<mtext>maps to</mtext>
</munderover>
<mi>Y</mi>
</mrow>
</mrow>
<mspace linebreak="newline" indentalign="center"></mspace>
<mrow style="border:1px;" dir="rtl">
<munderover style="border:1px">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<munderover style="border:1px" align="left">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<munderover style="border:1px" align="right">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<munderover style="border:1px" accentunder="true" accent="true">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
<mo style="border:1px">+</mo>
<mstyle displaystyle='false' >
<munderover style="border:1px">
<mo style="border:1px">∑</mo>
<mrow style="border:1px">
<mi>n</mi>
<mo>=</mo>
<mn>1</mn>
</mrow>
<mo style="border:1px">∞</mo>
</munderover>
</mstyle>
</mrow>
<mspace linebreak="newline" indentalign="center"></mspace>
<mrow style="border:1px;" dir="rtl">
<munderover style="border:1px;">
<mrow style="border:1px">
<mi>x</mi>
<mo>+</mo>
<mi>y</mi>
<mo>+</mo>
<mi>z</mi>
</mrow>
<mo style="border:1px">⏟</mo>
<mo style="border:1px">⏞</mo>
</munderover>
<munderover underspace="-0.1em" overspace="-0.1em" style="border:1px;">
<mrow style="border:1px;">
<mi>x</mi>
<mo>+</mo>
<mi>y</mi>
<mo>+</mo>
<mi>z</mi>
</mrow>
<mo style="border:1px;">⏟</mo>
<mo style="border:1px;">⏞</mo>
</munderover>
</mrow>
</math>