Usually I'd 100% agree. But I think the RethinkDB guys are a special case. They've gotten a huge amount done in the last few months -- in my "On Applying to Y

Usually I'd 100% agree. But I think the RethinkDB guys are a special case. They've gotten a huge amount done in the last few months -- in my "On Applying to YC" entry, they were the most notable exception to the "the teams that were moving the fastest at the beginning were also moving the fastest at the end". But if you remove YC from the equation, RethinkDB probably wouldn't have incorporated yet. (Well, let's pretend that they wouldn't have, at least.) They're just a few months in. Really, I think it's exceedingly rare when "co-founding" teams all start on the same day. There's a period where the company comes together over a few months and the people that are there when it incorporates become codified as the "co-founders". In this case, the RethinkDB guys are early enough on that I think some of that DNA is still being laid down, and the term co-founder isn't completely off.