# Length parameter for vertical skip space in a math environment?

I learned here that I can use \\[<len>] to explicitly set the vertical skip space between lines, e.g., <len> set to 3ex. I'd like to set <len> to, say, 2-times the normal length in this environment (e.g., align or dcases) but I don't know the length parameter that determines this.

Another example: I'd like to change the explicit 3ex below to some multiple of the vertical line-skip space in the align environment. Does anyone know what that is?

\documentclass{beamer}
\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\begin{frame}
Some sample text
\begin{align*}
\pi(\mu \mid x) &= \frac{\pi(\mu)\,\mathcal{L}(x \mid \mu)}{p(x)} \\
&= \begin{dcases*}
\frac{(1-w)\phi(x)}{p(x)} & for $$\mu=0$$ \\
\frac{w\gamma(\mu)\phi(x-\mu)}{p(x)} & for $$\mu \neq 0$$.
\end{dcases*}
\end{align*}
\end{frame}
\end{document}

-

use multiples of \baselineskip and in tabulars/arrays \normalbaselineskip

\documentclass{scrartcl}
\usepackage{mathtools}

\begin{document}

\begin{align*}
\pi(\mu \mid x) &= \frac{\pi(\mu)\,\mathcal{L}(x \mid \mu)}{p(x)} \\[\normalbaselineskip]
&= \begin{dcases*}
\frac{(1-w)\phi(x)}{p(x)} & for $$\mu=0$$ \\
\frac{w\gamma(\mu)\phi(x-\mu)}{p(x)} & for $$\mu \neq 0$$.
\end{dcases*}
\end{align*}

\end{document}

-
I noticed that using 2\baselineskip instead of either 3ex or 5\jot shrinks the spacing between the align environment and the text immediately preceding it. I'm doing this in a beamer document so that might be driving this result. –  lowndrul Oct 26 '11 at 17:12
then give a complete example not only a fragment of code –  Herbert Oct 26 '11 at 17:53
I completed the example I originally provided. –  lowndrul Oct 27 '11 at 1:30
I cannot see it. With \begin{spreadlines}{\normalbaselineskip} i get no other spacing –  Herbert Oct 27 '11 at 6:48

The linespacing in align and gather is defined by a length called \jot, though exactly what this value represents, I don't know.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{minipage}[t]{.45\textwidth}
\begin{gather}
a = b \\
c = d
\end{gather}
\end{minipage}
\begin{minipage}[t]{.45\textwidth}
\begin{gather}
a = b \\[2\jot]
c = d
\end{gather}
\end{minipage}


-
\jot represents a length for adjusting the spacing: the default is to add 1\jot to the baseline skip. –  egreg Oct 26 '11 at 16:41