LaTeX.org

I am using the \cline workaround suggested above, and have found that a large amount of vertical whitespace is inserted before the horizontal line, so I do this:

\begin{align*}
x + y &= 10 \\
x - y &= 5 \\[-11pt]
\cline{1-2}
x + y &= 10 \\
x &= 5 + y \\[-11pt]
\cline{1-2}
(5 + y) + y &= 10 ...