Phantom not working as expected in cases environment

Question

In the image below you can see the phantom not taking up as much width as it's expected in the cases environment:

Here's the MWE:

\documentclass{amsart}

\begin{document}
\begin{align*}
M+A+T & =12\\
\phantom{M+}A+T & =10
\end{align*}

\[
\begin{cases}
M+A+T & =12\\
\phantom{M+}A+T & =10
\end{cases}
\]

\end{document}

My questions are: why doesn't the phantom in the cases environment take up as much space as it does in the align* environment? How can I fix it?

I only have a temporary solution now, which is manually adding some \hspace.

Answer 1

Technically it does; it's the alignment difference between align and cases that's tripping you up. align has a RIGHT-LEFT alignment for elements around &, while cases has a LEFT-LEFT alignment. As such, the spacing shows (more) clearly in cases than align.

To achieve the proper spacing, using \phantom{M + {}} - add an empty group after + to ensure TeX sees it as a binary operator.

\documentclass{amsart}

\begin{document}

\begin{align*}
           M +     A + T & = 12 \\
  \phantom{M + {}} A + T & = 10
\end{align*}

\[
  \begin{cases}
             M +     A + T & = 12 \\
    \phantom{M + {}} A + T & = 10
  \end{cases}
\]

\end{document}

You are probably better off using the following cases:

\[
  \begin{cases}
             M +     A + T = 12 \\
    \phantom{M + {}} A + T = 10
  \end{cases}
\]

Answer 2

They do, but since you have aligned the two lines with & the space on left does not matter.

If you want the "Case" example to be aligned you have to add {} because + is a binary operator and Latex adds space between an operator and its arguments. Hence if there is no argument, as in your case, then no space is added.

\documentclass{amsart}

\[
\begin{cases}
M+A+T & =12\\
\phantom{M+{}}A+T & =10
\end{cases}
\]

\end{document}

Hope I could help :)

Answer 3

The difference is that align makes a pair of columns, one right aligned and one left aligned, so the \phantom does nothing in the case of align or align*.

In order to get the correct spacing you need an empty group: \phantom{M+{}}, because only in this way TeX will consider + as a binary operation symbol.

However there's a simpler solution:

\begin{cases}
\begin{aligned}[t]
M+A+T &= 12
A+T &= 10
\end{aligned}
\end{cases}

Full comparison:

\documentclass{amsart}

\begin{document}

\[
\begin{cases}
M+A+T = 12\\
\phantom{M+{}}A+T = 10
\end{cases}
\]

\[
\begin{cases}
\begin{aligned}[t]
M+A+T &= 12\\
A+T   &= 10
\end{aligned}
\end{cases}
\]

\end{document}

Answer 4

\documentclass{amsart}

\begin{document}

\[
  \left\{
    \begin{aligned}
      M+A+T &= 12\\
      A+T &= 10
    \end{aligned}
  \right.
\]

\end{document}