basic calls (such as "Right And Left Thru", or notably, "Circulate"). However, Challenge demands that the dancers have precise understanding of the call definitions, and combines and modifies calls in a way that yields more complex interpretations. Concepts In addition to the calls which denote a particular dance action, Challenge dancing makes extensive use of "concepts" which denote modifications to calls – either how they are executed or to whom they apply. There are only a few concepts before C1, e.g., the "As Couples" concept in Advanced. Each Challenge level adds about another ten new concepts. As with calls, the number of concepts at C4 is not as well defined. In addition, concepts can be nested, so that a call is modified by more than one. For example, "Tandem" means that two dancers one behind the other act as one dancer, while "Stable" means that the dancers are to move from place to place as the call would normally require but without changing their facing direction as they normally would. The caller can ask that a call be done "Stable Tandem" which means that pairs of dancers must act together to execute the call, but as they do this none of them turn. Formations Just as Plus adds Diamonds and Advanced adds Hourglasses, Challenge dancing adds new formations which have their own rules for circulation as well as calls that apply specifically to them. Additional formations include Butterflies, Galaxies, Triangles, Xs, and Zs. Also, many formations used at lower levels are extended to include cases where they are "distorted" – while they have the same number of dancers and the dancers follow the same rules of interaction, they are not spaced the same way. For example, a wave may be "offset" so that the two halves are not lined up with each other, or a "parallelogram" consisting of two lines offset from each other can be considered to be two "boxes" even though the dancers in each box are not adjacent to each other. Irregular distortions, e.g., where four arbitrary dancers not lined up in any obvious way are asked to act as a wave, may also be used. In each case, the dancers must follow the rules of the basic call, but adjust their actual movements so that they correspond to the positions of the distorted formation. Phantoms Challenge dancing makes extensive use of "phantoms" – imaginary dancers who