The <a href="https://en.wikipedia.org/wiki/Calkin%E2%80%93Wilf_tree" class="extiw" title="en:Calkin–Wilf tree">Calkin–Wilf tree</a>, drawn using an <a href="https://en.wikipedia.org/wiki/H_tree" class="extiw" title="en:H tree">H tree</a> layout. This tree contains each positive rational number exactly once; it is rooted at 1/1, and the two children of a/b are (a+b)/b and a/(a+b).
<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>