# Alignement problem underbrace within \align

I can't get the alignment correct with the underbrace:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\begin{document}

\begin{align}
\sin \alpha  = \frac{|CD|}{b} \text{ en } \sin \beta  = \frac{|CD|}{a} \\
&\underbrace {b \cdot \sin \alpha  = |CD| \text{ en } a  \cdot \sin \beta  = |CD|}_ \Downarrow \\
b  \cdot \sin \alpha  &= a \cdot \sin \beta \\
\frac{b}{\sin \beta} &= \frac{a}{\sin \alpha}
\end{align}

\end{document}

I think the correct way is to make the first two rules as one block, and they should be aligned with the downarrow on the equal sign.

My question is different than \underbrace and align environment because I want multiple lines above the underbrace that could be aligned one by one.

• Why don't you just use gather? – egreg Dec 21 '15 at 15:09
• @egreg I don't (didn't) know gather. I Started 2 years ago with align, and I'm using it ever since :-) – Arne Timperman Dec 21 '15 at 15:10
• Possible duplicate of \underbrace and align environment – bers Dec 21 '15 at 15:11
• @bers I've looked at that question before I started this question, but it doesn't give me an answer to my question ;-) – Arne Timperman Dec 21 '15 at 15:18

I don't think you want align in this case, because there's no “natural” alignment point; so gather seems a better choice.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\begin{document}

\begin{gather}
\sin \alpha  = \frac{|CD|}{b} \text{ en } \sin \beta  = \frac{|CD|}{a} \\
\underbrace {b \cdot \sin \alpha  = |CD| \text{ en } a  \cdot \sin \beta  = |CD|}_ \Downarrow \\
b  \cdot \sin \alpha  = a \cdot \sin \beta \\
\frac{b}{\sin \beta} = \frac{a}{\sin \alpha}
\end{gather}

\end{document}