"40", also known as "40 (How Long)", is a song by Irish rock band U2. It is the tenth and final track on their 1983 album, "War". The song is noted for its live performances; guitarist the Edge and bassist Adam Clayton trade instruments during performances of it, and as it was commonly played to end their concerts, the band would leave the stage one-by-one as the audience continued to sing the refrain "How long to sing this song?". The lyrics are a modification of the Bible's .