Rounded Corners of a Double Line

Question

Is there a way to round the corners of the inner line of a double line?

\documentclass[]{standalone}    
\usepackage{tikz}

\begin{document}
    \begin{tikzpicture}

        \draw [line width = 2pt, double distance = 6pt,
              rounded corners=3pt, line cap=round] (0,0) -- (3,4) -- (5,0);

    \end{tikzpicture}       
\end{document}

Answer 1

I don't know exactly how tikz computes the radii of the rounded corners here, but I guess that the radius you provide, 3pt would be the radius of a single line (my blue line). So the outer line radius need to be 3+3pt=6pt due to the separation and the inner line radius will be 3-3=0pt. If you change your radius for a bigger one the rounder corners are really rounded.

\documentclass[border=2mm]{standalone}    
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
  \foreach[count=\xi]\i in {6,10,...,22}
  {%
    \begin{scope}[shift={(5.5*\xi,0)}]
      \draw [line width = 2pt, double distance = 6pt,
             rounded corners=\i pt, line cap=round] (0,0) -- (3,4) -- (5,0);
      \draw [line width = 2pt, blue,
             rounded corners=\i pt, line cap=round] (0,0) -- (3,4) -- (5,0);
    \end{scope}
  }
\end{tikzpicture}