What is an alloy? The basic definition is simply a mixture of two items, although the term is most often used to refer to a mixture of two base metals. For example, brass is an alloy which is created by combining copper and zinc, while steel is an alloy which is created by combining carbon with iron.

The mixing of metals may provide certain advantages, such as increased strength, flexibility, or resistance to corrosion (rust). However, certain disadvantages may also exist; for example, the addition of carbon to iron to create steel increases the strength and corrosion resistance, but the alloy is also more brittle than pure iron. This is a problem that can be corrected with the addition of nickel or manganese to increase tensile strength.

Stainless steel is a common steel alloy obtained by adding chromium (and sometimes nickel) to the base "iron plus carbon" alloy. This has the advantage of being much more resistant to corrosive rust than plain carbon steel. Steel is the most widely recycled material in North America, since it is far cheaper to recycle steel than it is to mine iron to make new.

Aluminum alloys are created when aluminum is mixed with copper, zinc, manganese, silicon, or magnesium. These alloys are somewhat less resistant to corrosion than pure aluminum, but are more corrosion resistant than plain carbon steel, as well as being significantly lighter than carbon steel.

There are many copper alloys in addition to brass. Bronze is a copper alloy obtained by mixing copper with tin, aluminum, or silicon. Copper alloys also have a high degree of corrosion resistance. "Nickel silver" is actually a copper alloy, obtained by combining copper and nickel, and usually at least one additional metal, such as tin, zinc, lead, antimony, or cadmium. It is often used for cheaper grades of non-tarnishing "silver-plate" cutlery.

In addition, musical instruments (particularly woodwinds such as the oboe) are often made of nickel silver, because such instruments will not tarnish (as nickel silver is highly resistant to corrosion). Since the instruments do not require lacquer, they produce a brighter, more powerful sound quality.

Another common copper alloy is "cupronickel", a combination of copper, nickel, and either iron or manganese, which does not corrode in seawater. It is often used for marine hardware, such as propellers and hulls of work boats. It has also often been used for coinage, including the United States Jefferson Nickel, which is solid cupronickel (75% copper, 25% nickel), and the cladding on both sides of the United States’ half dollar coins (since 1971) and all quarters and dimes (since 1965).