Dr Turner, I once constructed a spreadsheet specifically for atDNA. Empirically I found the number to be closer to 10,000 than 1,000,000. I got into a discussion with someone(Tim Jansen?) about a related subject. The thing that we both agreed on was that the order of magnitude of the number should be around 10,000. I abandoned the effort as trying to keep the trees synchronized required more effort than the small, 10,000 tree benefit. So, if both parties have all 10,000 ancestors and descendants of those ancestors in both of their trees with no errors, the chances of find 3rd or even 4th cousin matches should be quite good. My main tree is 40,000 plus names and includes numerous tree fragments attached to various DNA and other potential matches. A periodic "janitorial" that resolves duplicate entries finds most of my matches. Sam ------------------------------------------------ "5. Even if I go back 4 generations, am I likely to randomly match any true 3rd cousins? NO!! because you need a database of over a million to find that one in a million 3rd cousin. Even if you have more than 300 3rd cousins alive today, there are over 300 million people living in the U.S. Does FTDNA test that many people? No."