Rank - amateur and professional
In theory, by virtue of their superior training and rigorous discipline, professional players are always stronger than amateurs. In practice, strong amateur 7 dans have occasionally beaten professionals in even games.
Andre Engels: In Europe, a 7D European rank is considered about equivalent in strength to a 1-2P professional.
BlueWyvern: From a conversation I had with a top European amateur, currently new Japanese pro's have the strength about of a 5 dan pro. Apparently the new crop has been getting tougher lately, so for someone of 1 dan strength, you need to take a pro who has been 1 dan for 10 or more years.
Calvin: Some data from the Pro-Am Honinbo Match might be interesting, although I can't find it compiled anywhere. Usually white gives komi and handicap. Here's a link where Iwai Ryuichi won against O Meien playing black with 2 stones and -5 komi. The result was B+4, so that's a two-stone game:
Harada Minoru, a very strong Japanese amateur, took three stones and -2.5 komi against Cho Chikun in the 1997 Pro-Am Honinbo and lost.
Bob McGuigan: It's interesting to see some of Harada's other results: 1963 and 64 defeated Sakata with 2 stones, 1968 defeated Rin with 3.5 reverse komi, 1979 jigo with Cho Chikun with 5 point reverse komi. The results of all these matches are in the Japanese yearbooks.
Sampi: Pros are just people who decided to make a career in go. An amateur can be as strong as a 9p, he just didn't pick up go professionally. Usually very strong amateurs become pros but thats not always the case.
A friend tells a story he got from his Japanese teacher. Not sure if the teacher saw it or just heard about it.
Japanese Go club, full of strong amateurs and several pros. Anyone under 5 dan is there for lessons. One regular is a well-known 9-dan pro. Everyone knows everyone else here, and most of the other strong players around.
One day, someone nobody recognises walks in, immediately sits down with the 9-dan, takes two stones, and loses by a narrow margin. Observers are amazed.
After he leaves, someone gets up the courage to ask "Sensei, who the devil was that?" "My elder brother. I learned Go from him and did not win a game against him until I was 18, but I turned pro and he went into business."