MathML generated by MathToWeb

A MathML example displaying the AMS-Latex subordinate environment aligned inside display environment equation.

\left. is used in place of \left\{, to insert a 'phantom delimiter' that does not appear, but which makes the necessary match with the right pairing delimiter \right\}. Recall that the \left and \right are the commands that cause the pairing delimiters \{ and \} to be automatically sized.
(MathToWeb also supports \right.).

Looking at the MathML below (the pertinent code is set in bold type), we see that <mphantom>...</mphantom> tags are used to accomplish this effect. Notice that the paring delimiter within these tags is a left parenthesis which is used to match a right curly brace. This is entirely acceptable, as any Latex left pairing delimiter can be matched with any Latex right pairing delimiter, and this is true for MathML also.

\begin{equation}
\tag{2.5}\label{equation-aligned-eg}
\left.\begin{aligned}
 y_0&=n_0+n_1\\
 z_0&=r_0+r_1
\end{aligned}\right\}
\qquad \text{$Y_0$ equations}
\end{equation}

<math xmlns="http://www.w3.org/1998/Math/MathML">
<mrow>
	<mphantom>(</mphantom>
	<mtable class="m-aligned">
		<mtr>
			<mtd columnalign="right">
				<msub>
					<mi>y</mi>
					<mn>0</mn>
				</msub>
			</mtd>
			<mtd columnalign="left">
				<mo>=</mo>
				<msub>
					<mi>n</mi>
					<mn>0</mn>
				</msub>
				<mo>+</mo>
				<msub>
					<mi>n</mi>
					<mn>1</mn>
				</msub>
			</mtd>
		</mtr>
		<mtr>
			<mtd>
				<msub>
					<mi>z</mi>
					<mn>0</mn>
				</msub>
			</mtd>
			<mtd columnalign="left">
				<mo>=</mo>
				<msub>
					<mi>r</mi>
					<mn>0</mn>
				</msub>
				<mo>+</mo>
				<msub>
					<mi>r</mi>
					<mn>1</mn>
				</msub>
			</mtd>
		</mtr>
	</mtable>
	<mo>}</mo>
	<mspace width="2.00em" />
	<mtext>
		<msub>
			<mi>Y</mi>
			<mn>0</mn>
		</msub>
	&nbsp;equations</mtext>
</mrow>
</math>