The ring is a tradition dating all the way back to ancient Egypt, beliving circles were symbols of eternity. The style was adopted by the ancient Romans and Greeks, who placed the ring on the fourth finger of the left hand because they believed that this finger contained the vena amoris, or the vein of love. 

Today, rings are expressively worn on any finger, but when it comes to love unions, very little has changed. It's quite a beautiful part of history.