Why is a wedding ring always worn on the third finger?

How do you wear an anniversary ring?
What is the correct placement for wearing wedding and engagement rings?
Which hand do you put an engagement ring on?
In modern western society, a woman's engagement ring is worn on her third finger until the day of her wedding where it is moved to her right hand during the nuptial ceremony. The groom then places her wedding band on her third finger during the vow exchange. After the ceremony, the bride usually moves her engagement ring in front of her wedding band. This symbolizes the fulfilled commitment between the bride and groom, as well as their eternal bond. During the wedding ceremony, the bride generally also places the groom's wedding band on his left third finger.
It is interesting to note that some medieval Christian wedding ceremonies placed the wedding ring on the thumb of the hand. After repeating "In the name of the Father, the Son, and the Holy Ghost," the ring was moved one finger at a time until it rested on the third finger