A jazz orchestra is a kind of musical group consisting of 12 to 25 musicians. It consists of a wooden section (saxophone, sometimes clarinets and flutes), a brass section (trumpets and trombones) and a rhythm section (piano, double bass, guitar and drums). It may contain some additional instruments such as vibraphone, various percussion and it may contain vocal soloists also. The music of the jazz orchestra has been arranged in advance and is not improvised on the spot. The improvised solos are played only when the arranger of the composition calls for it in the arrangement. It began its development in the first half of the 20th century. Fletcher Henderson and Duke Ellington were the two composers and arrangers who developed the genre most importantly. Jazz orchestras were most popular during the "swing" period, when over 400 different orchestras were active.