Brass is an alloy of copper and zinc, the proportions of zinc and copper can be varied to create a range of brasses with varying properties. By comparison, bronze is principally an alloy of copper and tin. Bronze does not necessarily contain tin, and a variety of alloys of copper, including alloys with arsenic, phosphorus, aluminium, manganese, and silicon, are commonly termed "bronze". Brass is a substitutional alloy. It is used for decoration for its bright gold-like appearance, for applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition casings and valves, for plumbing and electrical applications, and extensively in musical instruments such as horns and bells for its acoustic properties. It is also used in zippers. Brass is often used in situations where it is important that sparks not be struck, as in fittings and tools around explosive gases. The malleability and acoustic properties of brass have made it the metal of choice for musical instruments such as the trombone, tuba, trumpet, cornet, baritone horn, euphonium, tenor horn, and French horn which are collectively known as the brass within an orchestra.