hold positions in a formation that is only partially filled by real dancers. Challenge dancers must learn to do their part of a call in a particular formation even though many of the other dancers – possibly all of them – are missing. This requires both a clear understanding of the call definition and the ability to remember where the positions of the formation are without the usual visual and tactile cues. Calls which cause the dancers to change from one formation to another ("shape-changers") are especially challenging to dance with phantoms because the positions themselves will be changing and the entire formation may have to move to make room for other formations (even ones that have no real dancers at all to mark their position). Fractionalization Frequent use is made in Challenge dancing of fractionalized calls, both explicitly (e.g., the caller asks the dancers to do half of something) and implicitly through concepts. Many calls are defined to have specific "parts". Other calls can be broken down naturally based on the nature of the movement involved (e.g., a Hinge is half of a Trade). Starting at the C1 level, concepts are introduced which operate on call parts. For example, the "Finish" concept means to do all of the call except for the first part. At higher levels there are meta-concepts which involve applying concepts to only certain parts of a call (e.g., "Evenly" means that only the even-numbered parts of a call should be affected by the specified concept). To properly apply these concepts, the dancer must have both an excellent understanding of the definitions of the basic calls, including how the parts are defined, and the ability to keep track in real-time of what concepts are being applied to which parts. Challenge dancing organizations and events Clubs Some Challenge dancing is done in clubs similar to those at lower levels. In general, because of the limited number of dancers and callers, the higher the level the fewer clubs there will be. There are only a few cities that have clubs, with a live caller, dancing higher than C1 on a weekly basis. Tape groups Dancers who are interested in learning or dancing a particular Challenge level often form private groups and dance using recordings. Regional events In addition to local clubs, in many places there are events organized to offer Challenge dancing to the dancers of a larger