Alchemy symbols represent compounds and elements associated with the art of alchemy. These vary between alchemists, but some symbols are more popular than others; for example, the "tria prima" of Paracelsus in which a circle with a line through the middle symbolizes salt, a triangle on top of a cross symbolizes sulfur and a circle between a semicircle and a cross symbolizes mercury.
Other popular alchemy symbols include those of the Western Esoteric Tradition, in which triangles represent the four elements. A triangle symbolizes fire, an upside-down triangle symbolizes water, a triangle with a line through the center symbolizes air and an upside down triangle with a line through the center symbolizes earth. Another system is the planetary metal symbol in which the sun, the moon and five planets were associated with certain elements. In this system the sun is associated with gold, the moon with silver, Venus with copper, Mars with iron, Jupiter with tin, Saturn with lead and Mercury with mercury. Another common alchemy symbol is the Philosopher's Stone, which consists of a circle inside a square, inside a triangle, inside a circle. The top half represents the spiritual world and the bottom half represents the material world.