A sonata is a work in three or four movements (rarely two, or more than four) for a single instrument or a small number of performers. The term evolved through the history of music, designating a variety of forms prior to the Classical era. By the early 19th century the word came to define a multi movement work, almost always including a first movement in the so called 'sonata form', a second movement of a slow, contrasting character; a dance for third movement, and a finale in a rondo, theme and variations, or sonata form. The term 'sonata form' refers to the layout of the first movement of a sonata, which confronts two main themes of different quality, develops them and then recapitulates. This soon became a basic principle in academic music writing. It was applied to many forms of large works, and, together with the fugue, defined the composition and musical analysis procedures for almost a century. Today it is still widely studied, and sonatas continued to be written, even if musical languages have changed.