Most popular from the mid-14th through the early 16th century but used beyond this range, the buckler was a small metal shield fitted with a central metal boss and held by a strap in the fist. Wielded in combination with a sword, its small size of 16 inches (35.5 cm) or less posed little to no defense against arrows or bolts, but provided several other uses for the common soldier and knight.
The buckler’s main function was to protect the sword hand in close hand-to-hand fighting. Lightweight and versatile, the buckler proved a quick defense to block and deflect blows via the curved central boss. On the offense, it acted as a “metal fist”; its wielder could punch with it straight on or with the rim. In addition, the buckler could be used to bind the opponent’s weapon or grapple his arm, creating an opening to do further damage. And unlike wooden shields, the opponent’s sword would not get stuck in the metal.
While octagon, square, and trapezoid bucklers existed, round seems to have been the most popular shape. Some models featured a central spike, making the punch even more deadly, while others possessed hooks for snaring swords. Other small hooks at the edges are believed to have held lanterns, to light the bearer’s way at night.
Sword and buckler training was an established tradition in Europe. For example, English longbowmen carried bucklers and swords into battle just in case the fighting drew near. Yet, bucklers are a topic not generally documented, perhaps because the larger wooden shields carried the heraldry and thus got all the glory.
For an excellent source see http://www.thearma.org/essays/SwordandBuckler.htm