Also referred to as a parrying dagger, main gauche swords were used in juxtaposition with traditional rapiers during the late Middle Ages. The main gauche, which is French for left hand, was used to deflect incoming attacks while the rapier was utilized for offense. If the opportunity presented itself, the main gauche could also be used for offense, of course.
This combination of weaponry was particularly popular with the Schools of Fence in Renaissance Europe. As the sport of fencing evolved, the use of the offhand weapon fell out of style. Although the main gauche isn’t used in contemporary fencing, it still is a prized item for historical collectors.