According to the musicologist Walter Everett, "Sgt. Pepper" marks the beginning of McCartney's ascendancy as the Beatles' dominant creative force. He wrote more than half of the album's material while asserting increasing control over the recording of his compositions. In an effort to get the right sound, the Beatles attempted numerous re-takes of McCartney's song "Getting Better". When the decision was made to re-record the basic track, Starr was summoned to the studio, but called off soon afterwards as the focus switched from rhythm to vocal tracking. Much of the bass guitar on the album was mixed upfront. Preferring to overdub his bass part last, McCartney tended to play other instruments when recording a song's backing track. This approach afforded him the time to devise bass lines that were melodically adventurous – one of the qualities he especially admired in Wilson's work on "Pet Sounds" – and complemented the song's final arrangement. McCartney played keyboard instruments such as piano, grand piano and Lowrey organ, in addition to electric guitar on some songs, while Martin variously contributed on Hohner Pianet, harpsichord and harmonium. Lennon's songs similarly showed a preference for keyboard instruments.