6th Dan Black Belt.

row-koo-dahn) the sixth in a series; sixth degree black belt

Rokudan is an abbreviation for Rokudan no Shirabe, and it is one of Yatsuhashi Kengyo's famous pieces. It was originally a soukyoku, a kind of chamber music with the koto playing the leading part, but nowadays the part of the koto is more widely known than the original. The music is made from six columns (thus 'Roku' -- six, 'dan' -- columns), and there are exactly fifty-two beats in each column, except for the first row, which has four beats more.