Electrum, a natural alloy of silver and gold, was often used for making coins.

An alloy is a material composed of two or more metals or a metal and a nonmetal. An alloy may be a solid solution of the elements (a single phase), a mixture of metallic phases (two or more solutions) or an intermetallic compound with no distinct boundary between the phases. Solid solution alloys give a single solid phase microstructure, while partial solutions exhibit two or more phases that may or may not be homogeneous in distribution, depending on the thermal (heat treatment) history of the material. An inter-metallic compound has one other alloy or pure metal embedded within another pure metal.

Alloys are used in some applications, where their properties are superior to those of the pure component elements for a given application. Examples of alloys are steel, brass, pewter, phosphor bronze and an amalgam.

The alloy constituents are usually measured by mass. Alloys are usually classified as substitutional or interstitial alloys, depending on the atomic arrangement that forms the alloy. They can be further classified as homogeneous (consisting of a single phase), or heterogeneous (consisting of two or more phases) or intermetallic (where there is no distinct boundary between phases).