The fifteen different chord diagrams on six points (black line segments and blue dashed circles) or equivalently the fifteen different <a href="https://en.wikipedia.org/wiki/Perfect_matching" class="extiw" title="en:Perfect matching">perfect matchings</a> on a six-vertex <a href="https://en.wikipedia.org/wiki/Complete_graph" class="extiw" title="en:Complete graph">complete graph</a> (red vertices and yellow and black edges)
<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>