Briefly, this general psychological

Briefly, this general psychological principle is that particular perceptual and conceptual objects have special psychological status. This notion ap pears in a variety of contexts in the psychological literature, notably in the work of the Gestalt tradition and more recent work by Garner (1970), Rosch (1975), Rosch and Mervis (1975), and Goldmeier (1982). The basic idea is that within categories certain members are normative, unique, self-consistent, simple, typical, or the best exemplars of the domain (sometimes called "prototypes"). They are reference points to which other category members are compared. To illustrate with some examples that have been investigated experimentally, colors are often described with respect to "focal" colors, such as red, green, blue, and yellow. A color may be described as off-red or brownish-red, with implicit reference to a "focal" red. Similarly, numbers are rounded off to other numbers with special cognitive status, such as multiples of tens and hundreds. One says that 9 is almost 10, or that 95 is almost 100. Certain visual forms seem somehow "better" than others, because they are simpler, more regular, or more symmetric. The other, less regular forms are described as variants of these "good" forms. Thus, for example, a line may be described as almost vertical, and a quadrilateral figure as almost a square. These are all examples in which the elements are described in reference to certain other elements having special status within the category