One of the most commonly asked questions of beginners. How do ranks work in Go for amateurs?
Why do we start backwards? And why do we suddenly count upwards again? What is the difference between 9d and 9p? How is this related to rating? What are the basic differences between ranks used in different countries and different servers?
Let's start with the basics. Beginners start out as "30 kyu". As they get stronger, the number decreases to 29 kyu, 28 kyu ... down to 1 kyu. Kyu means "student" and is commonly abbreviated 'k'. The range from 10k to 30k is called DDK (double digit kyu) while 1k - 9k are SDK (single digit kyu).
The difference between each rank is supposed to be 1 stone of handicap. That is, an 8k should give 3 handicap stones to an 11k. All handicap games are played with komi of half a point. So an 4k playing a 3k plays black, not placing any additional handicap stones, and white receiving 0.5 komi.
Once a player surpasses 1k he becomes 1 dan, "master", abbreviated 'd'. Dan ranks go from 1 dan ('shodan') to 7 dan for amateurs (occasionally to 8 dan).
Professional players are obviously stronger than amateurs, since they studied the game very intensely for years and received training by pros. Only top amateurs have a chance of winning. Pro ranks go from 1 dan pro to 9 dan pro (abbreviated 'p' instead of 'd') with "10 dan" being a title that can be won in a tournament.
Nowadays, the difference in strength between pros is not related to their rank, because new 1 dan pros received the most up-to-date training and already possess vast knowledge (just like someone coming fresh from university is very up to date on the current research). Formerly, it was said the difference between 3 or 4 pro ranks is about 1 stone.
That being said, there are different ranking systems in different parts of the world that are not completely equal for various reasons. Online go serves got their own ranks as well. On Go servers, it is usually possible for amateurs to get assigned a rank of 9 dan.
While @mafutrct handled most of the essentials, I figured I would address some unanswered portions of your question.
Why do we start backwards? Well, this one is a little bit tough, and has more to do with the Japanese culture of ranks more than anything; but if I were to venture an educated guess, 級 (kyu) ranks count down because they are measurements of strength: an ordering if you will. Consider the following example:
Players ordered by strength
- Person A
- Person B
- Person C
In this example, we can understand that Person A, listed first, is the strongest of the three. Kyu ranks are similar to an ordering like this. So...
Why do we start to count upwards again? Well, like @mafutcrt mentioned, once you pass the highest rank, and become "dan" you are more like a master (this is the exact same system used in Japanese martial arts -- dan ranks are usually translated as "Nth degree black belt" in English). The Japanese character for "dan" (段) also means "stair", so each increase in rank can be thought of as one more step up the staircase of mastership.
What are the basic differences between ranks used in different countries and different servers? See this worldwide comparison of ranks on Sensei's Library.
So how is this all related to rating? By rating, in contrast to rank, I assume you mean strength, such as is represented by scores in the Elo rating system, among others. The simple answer is that in many systems, such as most online Go servers, there is little (if any) difference, because you can go down in rank just as well as you can go up. In others, such as with ranks given by the Japanese Go Association, you can never go down. In these systems, rank and rating are somewhat disconnected, but there is still generally some level of correlation. Professional ranks are very different, and are discussed in the question Are professional ranks representative for how many stones of handicap players can give each other?
"Expert" begins at 1 dan for amateurs. Then there's a second, higher level of expertise, 2 dan. One level above that is 3 dan, which is master. Then there are increasing levels of mastership rising to 4, 5, 6, and 7 dan. In recent decades, because of the popularity and training of go for amateurs, some of them became one to two levels stronger than 7 dan, call them 8 and 9 dan. A 9 dan amateur is almost as strong as a pro. All of these is based on being one stone stronger than the previous level.
Then the question is why do kyu ranks work in reverse order. A kyu player is a "non expert" player by definition. Then the ranks are based on "how far are they from expert?" A 1 kyu needs to rise only one level (stone) to become an expert, a 30 kyu, is 30 levels below. Think of kyu ranks as "minus" numbers (they're entered that way in the ratings computer), with 1 kyu being less "minus" than say, 10 kyu.
