This symbol appears thorough out medieval Europe on cathedrals, artwork, personal items, and jewelry, especially rings. The A and M characters in this symbol are thought to stand for Auspice Maria, or “Under Mary’s Protection”. It can appear with or without the bar in the a, though without the bar the symbol is more traditionally believed to be two M characters, with one inverted and superimposed over the other standing for Mater Misericordia, or “Mother of Mercy”.
Both symbols are monograms for Mary, the mother of Jesus, the founder of Christianity. They are have been use since at least the early medieval ages (c.1100) and have always been held in high regard as a protective symbol. Later they found their way onto religious and trade rings, collectively known as “Jesuit Rings”, on which they became one of the most commonly engraved symbols.
As the Auspice Maria, its use in Catholicism continues today as the symbol of the Sulpician Order, an order dedicated to the training of Catholic priests. However it can still be found in use by the devout on religious medals and other charms.