# Rounded Corners of a Double Line

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}


• Do you really want what you have drawn? -it will make the corner a lot thicker than the rest of the line! To preserve the line thickness in the corner, you need to increase the rounding radius as in the answer by @Juan Castaño. Your drawing can not be made by a single line(independent of double or not), but can be made by adding a second line under the first one. -Do you really want that? May 28, 2021 at 10:03
• @hpekristiansen Nope. But according to your comment, I can guess that radius of the inner and outer lines of a double line cannot be changed independently. May 28, 2021 at 10:05
• Yes. A double line is made by first drawing a really fat(black) line and then drawing a thin(white) line in the middle. If it was possible to change the inner and outer radius independent(it is not possible), the line width would change in the corner. May 28, 2021 at 10:08

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}


• Funny, I have guessed so initially and have tried to increase the round radius. Apparently, the radius I picked up was not large enough. May 28, 2021 at 10:01