 The majority of their recordings were produced by George Martin, who also played and composed string arrangements on multiple songs; his influence on the group led him to be referred to as the "Fifth Beatle".Between 1962 and 1968, the Beatles released their songs in both mono and stereo versions; "Abbey Road" and "Let It Be" were mixed and released only in stereo.Their songs often featured differences between the mixes and the group put the most effort into making the mono mixes.