or other instrumentalist. Rock music is traditionally built on a foundation of simple syncopated rhythms in a meter, with a repetitive snare drum back beat on beats two and four. Melodies often originate from older musical modes such as the Dorian and Mixolydian, as well as major and minor modes. Harmonies range from the common triad to parallel perfect fourths and fifths and dissonant harmonic progressions. Since the late 1950s, and particularly from the mid-1960s onwards, rock music often used the verse-chorus structure derived from blues and folk music, but there has been considerable variation from this model. Critics have stressed the eclecticism and stylistic diversity of rock. Because of its complex history and its tendency to borrow from other musical and cultural forms, it has been argued that "it is impossible to bind rock music to a rigidly delineated musical definition." In the opinion of music journalist Robert Christgau, "the best rock jolts folk-art virtues—directness, utility, natural audience—into the present with shots of modern technology and modernist dissociation". Unlike many earlier styles of popular music, rock lyrics have dealt with a wide range of themes, including romantic love, sex, rebellion against "The Establishment", social concerns, and life styles. These themes were inherited from a variety of sources such as the Tin Pan Alley pop tradition, folk music, and rhythm and blues. Christgau characterizes rock lyrics as a "cool medium" with simple diction and repeated refrains, and asserts that rock's primary "function" "pertains to music, or, more generally, noise." The predominance of white, male, and often middle class musicians in rock music has often been noted, and rock has been seen as an appropriation of black musical forms for a young, white and largely male audience. As a result, it has also been seen to articulate the concerns of this group in both style and lyrics. Christgau, writing in 1972, said in spite of some exceptions, "rock and roll usually implies an identification of male sexuality and aggression". Since the term "rock" started being used in preference to "rock and roll" from the late-1960s, it has usually been contrasted with pop music, with which it has shared many characteristics, but from which it is often distanced by an emphasis on musicianship, live performance, and a focus on serious and progressive themes as part of an ideology of authenticity that is frequently combined with an awareness of the genre's history and