Aikido Uniforms and Ranking

Aikido practitioners (commonly called aikidōka outside of Japan) generally progress by promotion through a series of "grades" (kyū), followed by a series of "degrees" (dan), pursuant to formal testing procedures. Most aikido organizations use only white and black belts to distinguish rank, but some use various belt colors. Testing requirements vary, so a particular rank in one organization is not always comparable or interchangeable with the rank of another. Some dojos do not allow students to take the test to obtain a dan unless they are 16 or older.

The uniform worn for practicing aikido (aikidōgi) is similar to the training uniform (keikogi) used in most other modern martial arts; simple trousers and a wraparound jacket, usually white. Both thick ("judo-style"), and thin ("karate-style") cotton tops are used. Aikido-specific tops are also available with shorter sleeves which reach to just below the elbow.

Most aikido systems also add a pair of wide pleated black or indigo trousers called a hakama. In many styles, its use is reserved for practitioners with black belt (dan) ranks or for instructors, while others allow all practitioners or female practitioners to wear a hakama regardless of rank.

Article source: http://en.wikipedia.org/wiki/Aikido