Two complementary <a href="https://en.wikipedia.org/wiki/Perfect_graph" class="extiw" title="en:Perfect graph">perfect graphs</a>. According to the <a href="https://en.wikipedia.org/wiki/Perfect_graph_theorem" class="extiw" title="en:Perfect graph theorem">perfect graph theorem</a>, the complement of any perfect graph is perfect.
<a href="//commons.wikimedia.org/wiki/User:David_Eppstein" class="mw-redirect" title="User:David Eppstein">David Eppstein</a>
<span class="int-own-work" lang="en">Own work</span>