A sign is a broad term encompassing a range of concepts. Symbols, indexes and icons are categories within this range. Each of these categories vary in their degree of abstraction. Symbols are the most abstract and are based on convention (everyone agrees on the meaning of the symbol). They don't look like the objects or concepts that they signify. Icons are at the other end of the spectrum. Icons do resemble the objects or ideas that they are referring to (the referent). Indexes fall between symbols and icons in their level of abstractness.

Distinctions are not often easy to make. A map for example, can be argued to be a symbol, an icon or an index with equal justification. Effective communication does not require us to differentiate between these types. An understanding of what sets them apart can, however, make our attempts at communication more effective.

A system of 50 symbol signs was designed for use at the crossroads of modern life: in airports and other transportation hubs and at large international events. Produced through a collaboration between the AIGA and the U.S. Department of Transportation, they are an example of how public-minded designers can address a universal communication need.
Prior to this effort, numerous international, national and local organizations had devised symbols to guide passengers and pedestrians through transportation facilities and other sites of international exchange. While effective individual symbols had been designed, there was no system of signs that communicated the required range of complex messages, addressed people of different ages and cultures and were clearly legible at a distance.

To develop such a system, AIGA and D.O.T. compiled an inventory of symbol systems that had been used in various locations worldwide, from airports and train stations to the Olympic Games. AIGA appointed a committee of five leading designers of environmental graphics, who evaluated the symbols and made recommendations for adapting or redesigning them. Based on their conclusions, a team of AIGA member designers produced the symbols.
A first set of 34 symbols was published in 1974, and received one of the first Presidential Design Awards; 16 more symbols were added in 1979. These copyright-free symbols have become the standard for off-the-shelf symbols in the catalogues of U.S. sign companies.