Traditionally in Japan the carp, swimming with the great power against the tide, is associated with boys.
Koinobori (carp banner) or Satsukinobori (may banner), are small flags, constructed like a windsock. They are hoisted in front of the houses in May for the Japanese boy-festival. Two large flags are hoisted for the parents and several small carp flags, a for every boy in the family. The large black flag stands for the father, the red for the mother and each blue for a son.