Graph Properties / Features

6. Two edges are called adjacent edges if they share a common endpoint (= common vertex).
   

   The edges $\{1,\color{red}{2}\}$ and $\{\color{red}{2},3\}$ are adjacent because they both contain vertex $\color{red}{2}$. Miriam Briskman, CC BY-NC 4.0.

   \documentclass[border=1pt]{standalone}
   \usepackage{tikz}
   
   \tikzset{
   vertex/.style={draw, circle, very thick, minimum size=1cm},
   edge/.style={very thick}
   }
   
   \begin{document}
   
   \begin{tikzpicture}
   
   \node[vertex] (1) at (0,0) {$1$};
   \node[vertex, red, dotted] (2) at (3,0) {$2$};
   \node[vertex] (3) at (6,0) {$3$};
   
   \draw[edge, red] (1) -- (2);
   \draw[edge, red] (2) -- (3);
   
   \end{tikzpicture}
   
   \end{document}