A buckle is a fastener that is typically used to secure a belt or a strap. They freqently have tongues and some also have throat plates running back from the buckle along the belt. Historically buckles could be made of bone, brass, bronze, silver and other metals. A single buckle could be be made of a combination of these materials, such as Viking buckles of copper alloy with iron tongues.
Softer metals do not function well in high load areas as they bend.
A wide variety of buckle shapes existed and there are examples of tongued single (one loop) buckles from at least the times of the ancient Greek. Double (two loops on either side of a central bar) buckles existed from at least the medieval era.