What does ampersand equals mean?

\begin{align}
E &= mc^2                              \\
m &= \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}
\end{align}


May I ask what does the &=do? Why is the ampersand needed?

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It means that the equations will be aligned at this point, just before the = sign. Placing the & before the = (or other operator) ensures correct spacing around the operator.

Here you have an example (ignore the actual mathemetics, I've just randomly added some numbers for displaying propuses)

\begin{align}
E*3*2*3 &= mc^2                              \\
m &= \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}
\end{align}


and without the &:

\begin{align}
E*3*2*3 = mc^2                              \\
m = \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}
\end{align}


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you might want to note that placing the & before the = (or other operator) ensures correct spacing around the operator. –  barbara beeton Jun 28 '13 at 14:35

They are not a combined symbol & is the alignment point in align just as it is in tabular It marks the division of the row into cells which are then stacked vertically. The second cell in each of those rows happens to start with = so Tex will typeset the rows with the = one directly above the other,

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