Common to pretty much all karate styles is the rank of the black belt. The belts that lead to black belt though, often vary widely from one school to another. The Goju based school I trained in for some years in had the following rank system leading to the Black Belt rank.

10th Kyu - White Belt
9th Kyu - White Belt, Blue Bar
8th Kyu - Blue Belt
7th Kyu - Blue Belt, Green Bar
6th Kyu - Green Belt 5th Kyu - Green Belt, Brown Bar
4th Kyu - Green Belt, Double Brown Bar
3rd Kyu - Brown Belt
2nd Kyu - Brown Belt, Black Bar
1st Kyu - Brown Belt, Double Black Bar
Shodan - Probationary Black Belt
1st Dan - Black Belt

Some schools would not have any blue or green belt rankings, and may have orange, yellow, purple etc. For karateka (people who practice karate) of different schools, a more meaningful understanding of another's rank might be gathered from knowing their kyu. Most styles have ten kyu, although some have eight.

The Colour Bar System

In between the gradings for full colour belts, are gradings like the green bar grading (other schools may also call this a green stripe grading, as it is indicated by a stripe of green material near the ends of the belt). Within my school, the bar gradings had an emphasis on technique, as compared to the strength component emphasised in a full colour belt grading. Sometimes the bar grades are honorary like blue bar being awarded to exceptional beginner students, although the belt awarded at one's first grading would be the full blue belt.
The ranking of double black bar is also a given rank and is regarded as an invitation to begin training for black belt.
This write up is mostly base in my own experiences and I'm sure there would be some contrasting accounts to this one.

It is also interesting to note that a karateka's introduction to 'in depth' training begins at black belt. I think there would be element of this common in many other martial arts, especially Japanese ones.

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