The Arming Sword was the sword of the Knights for 350 years. During its height of use a knight was considered undressed unless he was armed with the sword. Typically the sword was wielded with either a buckler or dagger in the opposite hand of the sword.
Even after the development and widespread use of the long sword hit Europe the Arming Sword was still carried by most knights. Where the long sword was eventually retired during the Renaissance, the Arming Sword instead evolved into the Rapier.
