An alloy is a substance formed from the combination of two or more metals or a metal with other non-metallic elements. Alloys are usually produced by melting the mixture of ingredients, and they can be classified into two categories based on the arrangements of atoms in their respective lattices: substitutional alloys and interstitial alloys. In substitutional alloys, the atoms of the alloying elements are similar in size and occupy the same sites the atoms of the base material would occupy in its lattice structure. In interstitial alloys, the atoms of the alloying elements are smaller than the atoms of the base material and occupy the spaces between the atoms of the base material. Alloys are known to feature metallic bonding, and they can be measured in terms of their mass percentage or atomic fractions. Alloys are used in a wide variety of industries because they combine different elements and contain atoms of different sizes, which gives them unique, useful properties. For example, alloy chemistry has contributed greatly to the strength, durability, and tonal quality of guitar strings/08%3A_Ionic_and_Metallic_Bonding/8.12%3A_Alloys). Some common examples of alloys include brass (copper and zinc), bronze (copper and tin), and steel (iron and carbon) .