A quartet is a group of four identical or similar objects, or or a grouping of four persons for a common purpose.

In music, a quartet is a method of instrumentation (or a medium) used to perform a musical composition, consisting of four parts. The most popular instrumentation is the string quartet.

Quartet also refers to a musical composition written for such a group.

A string quartet is a form of instrumentation in Western music tradition that consists of two violins, a viola, and a cello playing a multi-movement musical composition utilizing the sonata form. Another common instrumentation is the piano quartet, consisting of violin, viola, cello, and piano. The 'quartet' is a type of chamber music popular beginning in the 18th century, when public concert-giving began, towards present day, and is considered the most important form in chamber music. One of the first contributors, Luigi Boccherini, wrote 100 string quartets. Other important contributors are Joseph Haydn, Wolfgang Amadeus Mozart, and Ludwig van Beethoven.

The particular choice and number of instruments derives from the registers of the human voice: soprano, alto, tenor and bass. In the string quartet, two violins play the soprano and alto vocal registers, the viola plays the tenor register and the cello plays the bass register. In opera a quartet refers to the use of four voices.

Many believe the four instrument arrangement to be one of the most difficult to compose for compared with a trio or quintet, for instance. It is suggested that the difficulty arises from the complex interplay of four distinct voices performing under large temporal and tonal constrasts. Many of the performers find it one of the most enjoyable of performance arrangements, allowing each player to perform their own distinct part, rarely playing a "background" or "blending" part, important only in connection to the larger whole.

Later, in musical styles outside of the classical forms, many artists use the four part instrumentation such as in jazz saxophone quartets, guitar quartets, horn quartets, and barbershop quartets. The quartet form is constantly maturing and experimented with and is still in popular use.

Four humans sharing the same gestational period are called quadruplets and are usually all born within a short period of time.

In Middle-East politics, the Quartet is a group of four entities negotiating peace between Israel and the Palestinians. The four are U.S. Secretary of State Colin Powell, UN Secretary General Kofi Annan, Russian Foreign Minister Sergei Ivanov, and representatives of the European Union.

See also

• Trio
• Quintet