The custom of the bride standing to the left of the groom
at the altar came about during the Middle Ages, when
the groom had to have his right - or fighting - hand free
to fend off those who would try to prevent the marriage
ceremony from proceeding.
As legend has it, the medieval groom frequently had to defend his bride right up to the exchanging of vows from those who would try to prevent the marriage. To do so, he had to have his right arm, which held his sword or other weapon, ready for battle. Those who would try to stop the wedding ranged from jealous suitors to enraged fathers. Thus, the bride would stand to the left of her groom so that his right arm was free and unencumbered. This also is believed to be where the tradition of the "best man" came into being; he was the groom's backup to help fight off intruders to the wedding ceremony; back then, however, this person was chosen first and foremost because he was the best swordsman!