A three-page <a href="https://en.wikipedia.org/wiki/Book_embedding" class="extiw" title="en:Book embedding">book embedding</a> of the <a href="https://en.wikipedia.org/wiki/Complete_graph" class="extiw" title="en:Complete graph">complete graph</a> <i>K</i><sub>5</sub>. Because this is not a <a href="https://en.wikipedia.org/wiki/Planar_graph" class="extiw" title="en:Planar graph">planar graph</a>, it is impossible to embed it without crossings on a book of fewer pages.
<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>