There are no established conventions for, and few examples of, indexing visual material on the basis of its form. Most image databases use keywords to describe the form or function, and access data by text-based retrieval of these keywords. An image-based approach would order the data by appearance, e.g. Shepherd (1971) and Dreyfuss (1972). A taxonomy must be created in order to apply this technique to a new data set. Previous applications have been aided by certain limiting factors on the possible range of images indexed thus providing the key to a taxonomy, e.g. international pictorial signs in Dreyfuss. A fundamental principle which may be inferred from these studies is that the taxonomy needs to be related to the needs of the user, and not solely to abstract features of design. This is reflected in guidelines issued by the UK organisation The Society of Indexers. The studies also suggest the need for several approaches to classification within the same data set, e.g Dreyfuss, in which signs are located under more than one heading.