In Chinese opera, breath is based in the pubic region and supported by the abdominal muscles. Performers follow the basic principle that "Strong centralized breath moves the melodic-passages" ("zhong qi xing xiang"). Breath is visualized being drawn up through a central breathing cavity extending from the pubic region to the top of the head. This "cavity" must be under the performer's control at all times, and he or she develops special techniques to control both entering and exiting air. The two major methods of taking in breath are known as "exchanging breath" ("huan qi") and "stealing breath" ("tou qi"). "Exchanging breath" is a slow, unhurried process of breathing out old air and taking in new. It is used at moments when the performer is not under time constraint, such as during a purely instrumental musical passage or when another character is speaking. "Stealing breath" is a sharp intake of air without prior exhalation, and is used during long passages of prose or song when a pause would be undesirable. Both techniques should be invisible to the audience and take in only the precise amount of air required for the intended vocalization. The most important principle in exhalation is "saving the breath" ("cun qi"). Breath should not be expended all at once at the beginning of a spoken or sung passage, but rather expelled slowly and evenly over its length. Most songs and some prose contain precise written intervals for when breath should be "exchanged" or "stolen". These intervals are often marked by carats.