An mrow element is used to group together any number of sub-expressions, usually consisting of one or more mo elements acting as "operators" on one or more other expressions that are their "operands". MathML Fundamentals
<math mathsize="70"> <mspace linebreak="newline" indentalign="center"></mspace> <mtext style="border:1px;" mathsize="50">Mrow Test</mtext> <mspace linebreak="newline" indentalign="center"></mspace> <mrow style="border:1px;"> <mtext style="border:1px;">left to right</mtext> <mrow style="border:1px;"> <mtext>a</mtext> <mo>+</mo> <msup> <mi>X</mi> <msup> <mi>X</mi> <mn>2</mn> </msup> </msup> <mo>+</mo> <msup> <mi>X</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> </mrow> </mrow> <mspace linebreak="newline" indentalign="center"></mspace> <mrow style="border:1px;"> <mtext style="border:1px;">right to left</mtext> <mrow style="border:1px;" dir="rtl"> <mtext>a</mtext> <mo>+</mo> <msup> <mi>X</mi> <msup> <mi>X</mi> <mn>2</mn> </msup> </msup> <mo>+</mo> <msup> <mi>X</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> </mrow> </mrow> <mspace linebreak="newline" indentalign="center"></mspace> <mrow style="border:1px;"> <mtext style="border:1px;">row aligned top</mtext> <mrow style="border:1px;" rowalign="top"> <mtext>a</mtext> <mo>+</mo> <msup> <mi>X</mi> <msup> <mi>X</mi> <mn>2</mn> </msup> </msup> <mo>+</mo> <msup> <mi>X</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> </mrow> </mrow> <mspace linebreak="newline" indentalign="center"></mspace> <mrow style="border:1px;"> <mtext style="border:1px;">row aligned bottom</mtext> <mrow style="border:1px;" rowalign="bottom"> <mtext>a</mtext> <mo>+</mo> <msup> <mi>X</mi> <msup> <mi>X</mi> <mn>2</mn> </msup> </msup> <mo>+</mo> <msup> <mi>X</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> </mrow> </mrow> <mspace linebreak="newline" indentalign="center"></mspace> <mrow style="border:1px;"> <mtext style="border:1px;">row aligned axis</mtext> <mrow style="border:1px;" rowalign="axis"> <mtext>a</mtext> <mo>+</mo> <msup> <mi>X</mi> <msup> <mi>X</mi> <mn>2</mn> </msup> </msup> <mo>+</mo> <msup> <mi>X</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> </mrow> </mrow> <mspace linebreak="newline" indentalign="center"></mspace> <mrow style="border:1px;"> <mtext style="border:1px;">row aligned center</mtext> <mrow style="border:1px;" rowalign="center"> <mtext>a</mtext> <mo>+</mo> <msup> <mi>X</mi> <msup> <mi>X</mi> <mn>2</mn> </msup> </msup> <mo>+</mo> <msup> <mi>X</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mi>x</mi> <mn>2</mn> </msup> </mrow> </mrow> </math>