Why does a trailing newline (\\) change alignment of my equation?

I have 2 equations with two equality signs each. The equality signs should be vertically aligned. I can produce the expected output like this:1
Please read the footnote if you think this is just the wrong approach. You're right.

% Full MWE at the bottom of the question
\begin{align*}
foooo &= bar &\hspace{-3.6cm}&= baz \\
abc &= defgh  &\hspace{-3.6cm}&= xyz
\end{align*}


But if I just at a (seemingly) innocent \\ to the end of the second equation, the \hspace apparently has no effect anymore:

\begin{align*}
foooo &= bar &\hspace{-3.6cm}&= baz \\
abc &= defgh  &\hspace{-3.6cm}&= xyz \\
\end{align*}
%
\begin{align*}
foooo &= bar &&= baz \\
abc &= defgh  &&= xyz \\
\end{align*}


I used to think that \\ has no effect in the last line of an equation. Now, this example proves me wrong.

Question: I would like to understand why the trailing \\ changes the alignment in my equation / why it defeats the effect of \hspace{-3.6cm}.

I added an scrartcl tag because I realized that the behavior is different in article. My question is based on scrartcl.

MWE:

\documentclass{scrartcl}
\usepackage{amsmath}
\begin{document}
\begin{align*}
foooo &= bar &\hspace{-3.6cm}&= baz \\
abc &= defgh  &\hspace{-3.6cm}&= xyz
\end{align*}

\begin{align*}
foooo &= bar &\hspace{-3.6cm}&= baz \\
abc &= defgh  &\hspace{-3.6cm}&= xyz \\
\end{align*}
\end{document}


1 I know that I shouldn't be using negative whitespace as shown in my example but rather alignat:

\begin{alignat*}{2}
foooo &= bar &&= baz \\
abc &= defgh  &&= xyz
\end{alignat*}


However, the point of this question is not how to produce a certain alignment but to understand why the trailing \\ matters.

• The \\ adds another empty equation, and in this equation the second alignment is wider than in the first two lines: indeed, in these the width of the left hand side of the equation is negative, while in the last line it is zero. – GuM Jul 20 '18 at 12:52

The reason for the observed behaviour is not really the trailing \\, but rather the fact that there is a third line without \hspace{-3.6cm}. In the code below I have added such a line inbetween, so there is no trailing \\.

\documentclass{scrartcl}
\usepackage{amsmath}
\begin{document}
\begin{align*}
foooo &= bar &\hspace{-3.6cm}&= baz \\
abc &= defgh  &\hspace{-3.6cm}&= xyz
\end{align*}

\begin{align*}
foooo &= bar &\hspace{-3.6cm}&= baz \\
abc &= defgh  &\hspace{-3.6cm}&= xyz\\
\end{align*}
\begin{align*}
foooo &= bar &\hspace{-3.6cm}&= baz \\
&      &               &      \\
abc &= defgh  &\hspace{-3.6cm}&= xyz
\end{align*}
\end{document}