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>