Double Fantasy producer Jack Douglas claimed that since his Beatle days Lennon habitually tuned his D-string slightly flat, so his Aunt Mimi could tell which guitar was his on recordings. Occasionally he played a six-string bass guitar, the Fender Bass VI, providing bass on some Beatles numbers ("Back in the USSR", "The Long and Winding Road", "Helter Skelter") that occupied McCartney with another instrument. His other instrument of choice was the piano, on which he composed many songs, including "Imagine", described as his best-known solo work. His jamming on a piano with McCartney in 1963 led to the creation of the Beatles' first US number one, "I Want to Hold Your Hand". In 1964, he became one of the first British musicians to acquire a Mellotron keyboard, though it was not heard on a Beatles recording until "Strawberry Fields Forever" in 1967.