Length parameter for vertical skip space in a math environment?

Question

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{spreadlines}{3ex}
      \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{spreadlines}
  \end{frame}
\end{document}

Answer 1

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

\documentclass{scrartcl}
\usepackage{mathtools}

\begin{document}

\begin{spreadlines}{2\baselineskip}
  \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{spreadlines}

\end{document}

Answer 2

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}