# Phantom not working as expected in cases environment

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 :)

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:

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


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}