The title of Symphony No. 3, "Sinfonia Espansiva" (Op. 27, 1910–11), is understood by the English composer Robert Simpson to refer to the "outward growth of the mind's scope". It fully exploits Nielsen's technique of confronting two keys at the same time and includes a peaceful section with soprano and baritone voices, singing a tune without words. Symphony No. 4, "The Inextinguishable" (Op. 29, 1914–16), written during World War I, is among the most frequently performed of the symphonies. In the last movement two sets of timpani are placed on opposite sides of the stage undertaking a kind of musical duel. Nielsen described the symphony as "the life force, the unquenchable will to live".