An illustration of vertical separation in flight. 
I was flying from Los Angeles to New York in a passenger airliner (Boeing 767) at cruising altitude when this plane crossed our path underneath. The location is either the northwest corner of Arizona or near it in southern Utah, USA. As I had missed the same shot a few minutes before because I had waited until I *saw* the other plane, I was ready in case I got a second chance. I adjusted the camera for shutter speed priority and placed the camera against the window. I was lucky that a second plane passed underneath. As soon as I *heard* the other plane, I pushed the shutter release and the camera took the photo about half a second later.
<a href="//commons.wikimedia.org/wiki/User:Downtowngal" title="User:Downtowngal">Downtowngal</a>
<span class="int-own-work" lang="en">Own work</span>