The belt grading system in Judo allows all Judoka to have a clear understanding of what level they or their opponents are in the sport.
Juniors are graded in Mon, and Seniors in Kyu.
Juniors start as 1st Mon and go up to 18th Mon - Red belt to Brown belt and three tabs.
Seniors start at 6 Kyu and go up to 1st Kyu  - Red belt to Brown belt.
The next grade up for Juniors and Seniors is 1st Dan (Black Belt).  Juniors can only achieve this grade when they become 15  yrs old and have been a 1st Kyu for a minimum of six months.

All gradings up to and including Brown Belt are conducted within the club, but Dan gradings are regional and Judoka must fight other Brown Belts to achieve their Black Belt.  100 points are needed to achieve Black Belt status.  Normally, 10 points per winning fight, although it's possible to win a fight and only gain 7 points.
It's also possible to achieve your Black Belt in one grading, by winning two fights, and then being given a line up, (three more fights, one immediately after the other).  Win all five fights and you gain your Black Belt in a day.  Line ups can be achieved whether you're on zero points or if you've already earned up to 90 points.

What the BJA says:

The belt system is a distinctive feature of judo. Promotion within the Mon and Kyu grades is based on technical ability, knowledge and understanding, together with Japanese terminology and supplementary knowledge. There is no contest requirement but it is expected that the judoka follows a recommended randori and contest program as illustrated in the syllabus and has attained appropriate levels of achievement for each colour belt.

To attain a Dan grade, a judoka can enter an examination against other judoka of the same grade and, by demonstrating superiority over a cross-section of judoka at the same level, can win promotion to the next rank. Wins against judoka of the same grade or above in certain competitions may also count towards promotion. In this way, promotion through the Dan grades becomes increasingly difficult, since for each new grade the players will be of a higher standard. Judoka must also complete a competitive skills assessment that becomes progressively more demanding as they move through the Dan grades. However, a judoka may choose to follow the Technical Dan Grade pathway where they are examined on technical knowledge and understanding against a set criteria, higher than that for the contest route, and which also becomes progressively more difficult through the grades.

The time honoured distinction is between Kyu (student) and Dan (advanced) grades. Put simply, the distinction marks the point at which the emphasis shifts away from the acquisition of technique on to its effective application.

Promotion within the Dan grades does not carry a change of designated colour until 6th Dan is achieved. Even then it is customary for the holder to continue wearing a black belt in everyday practice, reserving the official colour for ceremonial occasions. The Dan grades are as follows:

Dan grades go from 1st Dan to 10th Dan, there is only one 10th Dan in the country.

Belt Colour
Traditional Name
1st Dan
2nd Dan
3rd Dan
4th Dan
shidan (yondan)
5th Dan
6th Dan
alternating red and white blocks
7th Dan
alternating red and white blocks
8th Dan
alternating red and white blocks
9th Dan
10th Dan

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