A symbol of the religion, the drum is perhaps the most sacred item in Vodou. Vodouists believe that ritual drums contain an etheric force, the "nam" or "nanm", and a spirit called "huntò". Specific ceremonies accompany the construction of a drum so that it is considered suitable for use in Vodou ritual. In the "bay manger tambour" ("feeding of the drum") ritual, offerings are given to the drum itself. Reflecting its status, when Vodouists enter the peristil they customarily bow before the drums. Different types of drum are used, sometimes reserved for rituals devoted to specific lwa; petwo rites for instance involve two types of drum, whereas rada rituals require three. In Vodou ritual, drummers are called "tambouriers", and becoming one requires a lengthy apprenticeship. The drumming style, choice of rhythm, and composition of the orchestra differs depending on which nation of lwa are being invoked. The drum rhythms typically generate a "kase" ("break"), which the master drummer will initiate to oppose the main rhythm being played by the rest of the drummers. This is seen as having a destabilising effect on the dancers and helping to facilitate their possession.