Challenge square dance, also known as Challenge dancing, is modern western square dance focused on the puzzle-solving aspects of square dancing at the most difficult or "challenging" levels. In modern square dancing, the dancers interpret the caller's directions (calls) to determine whom to act with, where to go, and how to get there. All square dancers learn standard "definitions" of calls, which they recall and use when the caller issues a given command. Most square dancers learn about 100 calls. Challenge dancing involves many more calls, and the definitions and choreographic combinations are more abstract and difficult to process. The same physical movements (e.g. arm turns, passing by) are employed, but deciphering the complex commands and figuring out what you're supposed to do is much more challenging. Levels and lists Modern western square dancing is organized into a hierarchy of standard syllabus programs (also called "levels"). Each level is described by a list of the calls that will be used, inclusive of all the material from the lower levels. The higher the program, the more material (i.e. calls) on that list, and the greater ability to recall and handle complexity is required of the dancers. The levels preceding Challenge are: Mainstream, Plus, and Advanced (A1 and A2). There are five Challenge levels: Basic Challenge (C1), Extended Challenge (C2), Extended Challenge (C3A), Challenge 3B (C3B) and Challenge 4 (C4). The first three of these are maintained and organized by Callerlab, while the last two exist on a more ad hoc basis, maintained by a small set of interested callers. As a result of this there are a few different C4 lists, mostly the same, but with various differences. What makes Challenge dancing challenging? C1 dancers must be familiar with about 250 calls (including those from Mainstream, Plus, and Advanced). By C3, the dancers must be familiar with more than 400 calls. C4 dancers typically know at least 700 calls. Challenge square dancing not only involves knowing many more calls, but is typified by the extensive use of Concepts, abstract and physical formations, "Phantom" dancers, and call deconstruction (such as Fractionalization). Some square dance calls are more complex than others, involving more series of actions and more abstract identifications of whom to do the action with. Challenge lists include more of these complex calls, but none are by themselves more complex than calls at the Advanced level. Much of Challenge dancing uses