Artistic device denoting spiritual devotion
The mandorla (Italian: almond) is used in religious art (primarily Christian, but also in some Buddhist art) to denote holiness. It is best thought of as a full-body halo, and represents the aureola or radiance of holy people, most especially the Christ.
It first appeared during the 5th century, as a decorative feature in the mosaics in the Church of Santa Maria Maggiore in Rome, but was quickly adopted by other artists as a standard technique to show the sanctity of the Madonna and the Saints.
Following the growth of the more naturalistic artistic portrayals after the 15th Century, the mandorla fell from grace, although has come back into favour in some church art (often as a vesica piscis, the overlapping of two circles), as representing the bringing together or overlapping of the physical and spiritual sides of human life.
In the Eastern tradition, it was used to show how spiritual life develops.
"Starting from a point, two lines, which have a curving path, separate like the two branches which grow out of a single stem. But the curving path of these separating directions, have a tendency not only to go further and further apart, but to eventually come back together again, once more discovering a common point of meeting."
The vesica predates the mandorla by some hundreds of years - certainly in ancient Greece, possibly first among the Pythagoreans, who considered it a holy figure.
The mathematical ratio of its width (measured to the endpoints of the "body", not including the "tail") to its height was reportedly believed by them to be 265:153. This ratio, equal to 1.73203, was thought of as a holy number, called the measure of the fish.