13 plane trees with four ordered leaves, with all root-leaf paths the same length and with different numbers of nodes at each level. The gaps between leaves are labeled by the height  above the leaves of the <a href="https://en.wikipedia.org/wiki/Lowest_common_ancestor" class="extiw" title="en:Lowest common ancestor">lowest common ancestor</a> of the two adjacent leaves; these heights induce a <a href="https://en.wikipedia.org/wiki/Weak_ordering" class="extiw" title="en:Weak ordering">weak ordering</a> of the gaps. As shown by <a href="https://en.wikipedia.org/wiki/Arthur_Cayley" class="extiw" title="en:Arthur Cayley">Arthur Cayley</a> in 1859, the trees of this type are counted by the <a href="https://en.wikipedia.org/wiki/ordered_Bell_number" class="extiw" title="en:ordered Bell number">ordered Bell numbers</a>.
<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>