In Icons of Rock: An Encyclopedia of the Legends Who Changed Music Forever, Scott Schinder and Andy Schwartz describe the Beatles' musical evolution:  In their initial incarnation as cheerful, wisecracking moptops, the Fab Four revolutionised the sound, style, and attitude of popular music and opened rock and roll's doors to a tidal wave of British rock acts. Their initial impact would have been enough to establish the Beatles as one of their era's most influential cultural forces, but they didn't stop there. Although their initial style was a highly original, irresistibly catchy synthesis of early American rock and roll and R&B, the Beatles spent the rest of the 1960s expanding rock's stylistic frontiers, consistently staking out new musical territory on each release. The band's increasingly sophisticated experimentation encompassed a variety of genres, including folk-rock, country, psychedelia, and baroque pop, without sacrificing the effortless mass appeal of their early work.  In The Beatles as Musicians, Walter Everett describes Lennon and McCartney's contrasting motivations and approaches to composition: "McCartney may be said to have constantly developed - as a means to entertain - a focused musical talent with an ear for counterpoint and other aspects of craft in the demonstration of a universally agreed-upon common language that he did much to enrich. Conversely, Lennon's mature music is best appreciated as the daring product of a largely unconscious, searching but undisciplined artistic sensibility."  Ian MacDonald describes McCartney as "a natural melodist - a creator of tunes capable of existing apart from their harmony". His melody lines are characterised as primarily "vertical", employing wide, consonant intervals which express his "extrovert energy and optimism". Conversely, Lennon's "sedentary, ironic personality" is reflected in a "horizontal" approach featuring minimal, dissonant intervals and repetitive melodies which rely on their harmonic accompaniment for interest: "Basically a realist, he instinctively kept his melodies close to the rhythms and cadences of speech, colouring his lyrics with bluesy tone and harmony rather than creating tunes that made striking shapes of their own." MacDonald praises Harrison's lead guitar work for the role his "characterful lines and textural colourings" play in supporting Lennon and McCartney's parts, and describes Starr as "the father of modern pop/rock drumming".

Answer this question "What is notable about their music style?" by extracting the answer from the text above.
". His melody lines are characterised as primarily "vertical", employing wide, consonant intervals which express his "extrovert energy and optimism".