A symphony is a multi movement piece of music, most often written for a large instrumental force. The term has varied in meaning since its origins in ancient Greece. During the Baroque period, the terms symphony and sinfonia (its equivalent in some romance languages) were used to describe several instrumental forms, usually part of larger works (such as three part inventions). During the 18th century, the term symphony was often interchangeable with the term 'overture'. In fact, the fast-slow-fast structure of the Italian overture, together with the sonata form, exerted a big influence on the development of the 'traditional' or Romantic symphony form, which ultimately came to be defined as: an opening allegro movement in sonata form, a slow movement, a dance movement, and a second fast movement, usually a rondo or a set of theme+variations. During the Romantic period composers experimented with variations on this layout, and included programmatic elements into their symphonic compositions.