Depends as others have said on the organization. For the USAF (United States Aikido Federation), Dan testing is very strict, and has strict requirements. It has to be EITHER a Shihan member of the USAF Technical Committee (there aren't many of them) or TWO shidoin (in our organization, heads of dojos) that conduct the test, and they can only test up to the rank below them. If you want to test above that, you need to find someone above that rank..... In Aikikai Aikido, there was only one 10th Dan ever, and that was Koichi Tohei. There are some 9th Dan, but they are rare, and when they are promoted, it is by the head of the Aikikai-Doshu. Doshu has no rank, he is simply Doshu. O'Sensei being the founder, had no rank. But, as others have said, it varies considerably by art and more importantly organization. The ranks really don't mean anything in my opinion anyway, besides being a measuring stick for your own progress......I've actually practiced with an aikido practitioner that has been practicing for over 20 years. He is as good as any 4th or 5th Dan I've been with, and yet, he stopped testing at 2nd kyu. Said that he didn't really see the point as that was not why he practiced...he just wants to rock and roll, learn, and have fun. Could care less about grades....