The Romans called it the "vena amoris" - the vein of love. They believed a vein ran directly from the fourth finger of the left hand to the heart, making it the appropriate finger for a ring that symbolised love and commitment.
The belief was anatomically incorrect. All fingers have comparable circulatory connections to the heart. The vena amoris doesn't exist as a distinct structure.
The belief was symbolically durable.
It spread through Europe with the Roman Empire and survived the Empire's fall. Across centuries of dramatically different fashion, religion, and custom, the basic placement of the wedding or engagement ring remained: fourth finger, left hand, in most Western cultures.
What's interesting is that the tradition persists even among people who know the anatomy is wrong - who know there's no special vein, who understand that the finger choice is an accident of ancient misunderstanding. Knowing this doesn't prompt people to move their wedding ring to a different finger. Because the meaning is no longer carried by the anatomy. It's carried by the shared recognition of what the placement means.
In Eastern Europe, Russia, Greece, and several other cultures, the right hand is used instead - a different convention, equally strong, for reasons rooted in different historical traditions.
The cultural variation is worth noting: there's nothing universal about left versus right. The finger choice is convention. Conventions are local and historical, not necessary. What's universal, perhaps, is the impulse to mark commitment on the body in a visible, permanent way.
The finger matters. Which finger is negotiable.
