# Marking a specific point on an arc

I want to draw a line from one arc to other but I do not know if it is possible to mark a point on an arc and start drawing on that point. Is it possible to draw lines like below image?

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\draw[thick] (90:3cm) arc (90:0:3cm);
\draw[thick] (90:3cm) arc (180:90:3cm);
\end{tikzpicture}

\end{document}

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This is fixed in the CVS version and can be done by putting nodes and coordinates but for the time being you can use the marking decoration to mark the arc path with a node. –  percusse Dec 24 '12 at 0:56

There are a lot of possibilities with tikz.

## First method

Below is one if you want to specify the points manually.

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\draw[thick] (90:3cm) arc (90:0:3cm);
\draw[thick] (90:3cm) arc (180:90:3cm);
\coordinate (A) at (80:3cm);
\path (90:3cm) arc (180:135:3cm) node [inner sep=0pt] (B) {};
\draw [blue,thick] (A) -- (B);
\foreach \X in {A,B}
\fill [blue] (\X) circle (2pt);

\coordinate (C) at (33:3cm);
\path (90:3cm) arc (180:123:3cm) node [inner sep=0pt] (D) {};
\draw [red,thick] (C) -- (D);
\foreach \Y in {C,D}
\fill [red] (\Y) circle (2pt);
\end{tikzpicture}

\end{document}


## With decorations.markings library

And here is a solution using decorations.markings library.

%http://tex.stackexchange.com/questions/88053/marking-a-specific-point-on-an-arc/88060#88060
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[
every node/.style={circle,inner sep=1.5pt,outer sep=0pt},
decoration={
markings,% switch on markings
mark=at position 1cm with {\node (A) [fill=blue] {};},
mark=at position 3cm with {\node (B) [fill=red] {};},
mark=at position 0.7 with {\node (C) [fill=blue] {};},
mark=at position 0.8 with {\node (D) [fill=red] {};}
}
]

\draw [thick,postaction={decorate}] (90:3cm) arc (90:0:3cm)(90:3cm) arc (180:90:3cm);
\draw [blue,thick] (A) -- (C);
\draw [red,thick] (B) -- (D);
\end{tikzpicture}
\end{document}


The advantage of the second method is that you can specify the placement of the points relatively (0.1, 0.23 and so on) between the first coordinate/node and the last coordinate/node of your path. So this may apply even to arbitrary paths.

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