Noh is the classical theatre of Japan which was codified in the 14th century by the father and son actors Kan'ami and Zeami under the patronage of the Shogun (supreme military leader) Ashikaga Yoshimitsu. Under Yoshimitsu the Zen principles of restraint, understatement, economy of movement and frugality of expression became incorporated into the performance. By the early seventeenth century Noh had become an even more austere and formalised drama reserved almost exclusively for the Tokugawa family, the ruling military elite.

The mask of Koujijo (or perhaps that of Kojo) depicts an the old man with grave and noble features. The mask is used to represent a god in human form and is used in 'Takasago', one of the most well known Noh plays of the 'God' (First) Group to represent the spirit of the sacred pine tree of Sumiyoshi. In this play the mask is paired with an actor wearing the mask of an old woman, Uba (see 578K-1886) which represents the spirit of the Takasago pine tree. Although separated by both time and space the spirits of the two trees are irrevocably bonded.

The mask has elegant refined features and radiates a profound sense of dignity and age. It is unexpectedly light and is carved from a single block of hinoki (Japanese cypress) onto which a layer of gofun (gesso) has been applied before the mask was painted. Much of the original hair has been lost, particularly around the chin. The age and respect shown to this mask is indicated by the evidence of many repairs and by the fact that the interior has been strengthened by layers of textile soaked in lacquer.