For the backing tracks, Wilson used an ensemble that included the classically trained session musicians frequently employed on Spector's records, a group later nicknamed "the Wrecking Crew". Wilson had been employing the services of session musicians due to the increasingly complex nature of his arrangements and because his bandmates were often away playing concerts. Carl, who had occasionally played guitar alongside these musicians at Brian's sessions, commented that his contributions were not as significant as before and that "It really wasn't appropriate for us band to play on those ["Pet Sounds"] dates—the tracking just got beyond us."