The measure of the fish was a formula known to Archimedes in the third century BC. It is derived from two overlapping circles where the overlap is known as the vesica piscis, the bladder of a fish, from its similarity to the shape of a fish. It is also known as the mandorla, Latin for almond.

Inside this intersecting area, in addition to the triangle, the tetrad, the square, the pentacle, and many more polygons, the width to height ratio is 265:153 and, geometrically, the ratio of these dimensions is the square root of 3.

Symbolically, one sphere represented the Supreme Being, the second circle the goddess and the overlap, with its obvious similarity to the vulva, the creation of their offspring.

Early Christians used the symbol as a method to describe the coming together of heaven and earth, between the divine and human, or spirit and matter. They also used it to make themselves known to each other by scraping two lines indicating a stylised fish on a wall. One would make a small circle and the next to pass by would make a another slightly overlapping circle thus completing the vesica piscis.