I hope I'm not duplicating anything here...

My simple question is: what are the rules to align the various lines in an align environment?

For example - at first I thought that

```
\begin{align*}
a &= bbbbbbbbbbbbbb \\
c &= d
\end{align*}

```

would align the "=" at the center of the page, but that's not the case.

Then I tought that it would put the longer (first) equation in the middle of the page, but that's not the case either. In fact the previous code snippet would not put the "=" in the same position as

```
\begin{align*}
a &= bbbbbbbbbbbbbb \\
cc &= d
\end{align*}

```

(only difference is that there are two "c"'s now).

So what's the rule?