Nail fetishes are a unique and important phenomenon of Kongo sculpture. In the Kongo, all these fetishes are called nkisi. Nkisi means “medicine”. Historically, there were two types of nkisi, public and private, with some having vital democratizing roles, as sources of empowerment for rural residents and individuals outside the court. In order to face certain trials (illness, conflict, sterility, sorcery, etc), the Kongo take recourse in this type of object. Inserting nails or pieces of metal into the nkisi to tie down an agreement or lawsuit is called bibaaku. It activates the figure to bring destruction to those who do not fulfill their end of the pact, oath, or treaty. Thus they attempt to resolve their problem, or to intimidate or punish the individual responsible for it. An nkisi is made by a sculptor who works together with a nganga (ritual expert). The sculptor carves a male figure. Next, the nganga completes the figure by placing the ingredients that have positive or negative power in an abdominal cavity made by the sculptor. These ingredients are ‘carriers’ for a variety of magical objects, which can be sand, roots or leaves, or other objects. Keeping track of the precise history of each insertion was the duty of the nganga. Each nail in Nkisi represents a petition to the force contained in the figure.
The 2014 "Kongo Exhibit" at the Princeton University Art Museum displayed a number of great NKISI. They were appropriately labeled as figures of "conspicuous aesthetic force".