The phrase chamber music is now used to mean a piece of music written by a composer for a small musical ensemble in which no two instruments play the same music. It is opposed to orchestral music or opera, for example.
Originally, the phrase meant any kind of music designed to be played in a private room rather than a concert hall, a church or a theatre. In this sense, the madrigalss of the renaissance period in the 16th century may be considered chamber music.
When the phrase is used today, however, it is usually in reference to purely instrumental, non-vocal music. The most prominent Baroque form of this type is the trio sonata. In the Classical period, new forms were developed, most importantly the string quartet. These pieces were often written for amateurs, and not intended to be played in public. Many of the string quartets of Joseph Haydn and Wolfgang Amadeus Mozart, for example, were written to be played for fun and in private, by a string quartet of which they were both members.
One of the composers responsible for bringing chamber music to the concert hall is Ludwig van Beethoven. He wrote chamber music for amateurs, such as the Septet of 1800, but his last string quartets are very complex works which amateurs would have struggled to play. They are also seen as pushing the boundaries of acceptable harmony of that time, and are regarded as some of his most profound works. Following Beethoven in the romantic period, many other composers wrote pieces for professional chamber groups.
Chamber works exist for many different combinations of instruments, with the string quartet often seen as the most important. Popular chamber groups other than the string quartet include the string trio, the piano trio, the piano quintet and the string quintet. Woodwind instruments and brass instruments are used less often. Several composers have written works for mixed groups of wind and strings, and some have written for wind instruments alone, but with the exception of the French horn, brass instruments are very rarely used. This is probably in part due to the fact that at the time chamber music was first being written, brass instruments did not have valves, and so could only produce a limited number of notes.
The phrase chamber music suggests a piece for at least two instruments, but there is no theoretical upper limit to the number of instruments. In practice, chamber works for more than eight instruments are rare.
It should be noted that while chamber music is frequently played in public concerts, it is usually heard in halls much smaller than those used for orchestral concerts. The more intimate acoustics of a smaller space, imitating the drawing rooms in which such music was originally played, are more suitable for a small group of instruments.