Paul Tillich argued that, while signs are invented and forgotten, symbols are born and die.  There are, therefore, dead and living symbols. A living symbol can reveal to an individual hidden levels of meaning and transcendent or religious realities. For Tillich a symbol always "points beyond itself" to something that is unquantifiable and mysterious; symbols open up the "depth dimension of reality itself".  Symbols are complex, and their meanings can evolve as the individual or culture evolves. When a symbol loses its meaning and power for an individual or culture, it becomes a dead symbol. When a symbol becomes identified with the deeper reality to which it refers, it becomes idolatrous as the "symbol is taken for reality." The symbol itself is substituted for the deeper meaning it intends to convey. The unique nature of a symbol is that it gives access to deeper layers of reality which are otherwise inaccessible. 
All parts of the biohazard sign can be drawn with a compass and straightedge . The basic outline of the symbol is a plain trefoil , which is three circles overlapping each other equally like in a triple Venn diagram with the overlapping parts erased. The diameter of the overlapping part is equal to half the radius of the three circles. Then three inner circles are drawn in with 2 ⁄ 3 radius of the original circles so that it is tangent to the outside three overlapping circles. A tiny circle in center has a diameter 1 ⁄ 2 of the radius of the three inner circles, and arcs are erased at 90°, 210°, and 330°. The arcs of the inner circles and the tiny circle are connected by a line. Finally, the ring under is drawn from the distance to the perimeter of the equilateral triangle that forms between the centers of the three intersecting circles. An outer circle of the ring under is drawn and finally enclosed with the arcs from the center of the inner circles with a shorter radius from the inner circles. 
Other linguists will argue that the @ sign is a more recent development, appearing sometime in the 18th century as a symbol used in commerce to indicate price per unit, as in 2 chickens @ 10 pence. While these theories are largely speculative, in 2000 Giorgio Stabile, a professor of the history of science at La Sapienza University in Italy, discovered some original 14th-century documents clearly marked with the @ sign to indicate a measure of quantity - the amphora, meaning jar. The amphora was a standard-sized terra cotta vessel used to carry wine and grain among merchants, and, according to Stabile, the use of the @ symbol ( the upper-case "A" embellished in the typical Florentine script) in trade led to its contemporary meaning of "at the price of."