A key role in Palo is played by a vessel, sometimes a clay pot or gourd, at other times an iron pot or cauldron, the "calderos", which is often wrapped tightly in heavy chains. This is the "nganga", a term which in Central Africa referred to a man who oversaw religious rituals. It is also known as the "prenda", a Spanish term meaning "treasure" or "jewel." It is alternatively sometimes called "el brujo" ("the sorcerer"), or the "cazuela", while a small, portable version is termed the "nkuto". The terms "prenda" and "nganga" designate not only the vessel but also the spirit believed to inhabit it. The choice of nganga vessel can be determined depending on mpungu. The nganga is deemed to be alive, and is regarded as the source of the Palo practitioner's supernatural power. The practitioners are termed the "perros" (dogs) or "criados" (servants) of the nganga.