limited form or not at all. Although they are usually referred to as Dorling cartograms after Daniel Dorling's 1996 algorithm first facilitated their construction, these are actually the original form of cartogram, dating back to Levasseur (1876) and Raisz (1934). Several options are available for the geometric shapes: Circles (Dorling), typically brought together to be touching and arranged to retain some semblance of the overall shape of the original space. These often look like proportional symbol maps, and some consider them to be a hybrid between the two types of thematic map. Squares (Levasseur/Demers), treated in much the same way as the circles, although they do not generally fit together as simply. Rectangles (Raisz), in which the height and width of each rectangular district is adjusted to fit within an overall shape. The result looks much like a treemap diagram, although the latter is generally sorted by size rather than geography. These are often contiguous, although the contiguity may be illusory because many of the districts that are adjacent in the map may not be the same as those that are adjacent in reality. Because the districts are not at all recognizable, this approach is most useful and popular for situations in which the shapes would not be familiar to map readers anyway (e.g., U.K. parliamentary constituencies) or where the districts are so familiar to map readers that their general distribution is sufficient information to recognize them (e.g., countries of the world). Typically, this method is used when it is more important for readers to ascertain the overall geographic pattern than to identify particular districts; if identification is needed, the individual geometric shapes are often labeled. Mosaic cartograms In this approach (also called block or regular cartograms), each shape is not just scaled or warped, but is reconstructed from a discrete tessellation of space, usually into squares or hexagons. Each cell of the tessellation represents a constant value of the variable (e.g., 5000 residents), so the number of whole cells to be occupied can be calculated (although rounding error often means that the final area is not exactly proportional to the variable). Then a shape is assembled from those cells, usually with some attempt to retain the original shape, including salient features such as panhandles that aid recognition (for example, Long Island and Cape Cod are often exaggerated). Thus, these cartograms are usually homomorphic and at least partially contiguous. This