Research into the charcteristics of R'n'B:

Musical Rhythm is the most important distinguishing characteristic of R&B music and its sub-styles.  While all genres of R&B typically depend upon a four beat metre and employ a backbeat (accenting each bar on beats two and four, which are usually unaccented), the specific approach to the expression of musical time (the so-called groove - the characteristic rhythms employed within the basic metre) is one of the primary means of differentiating one genre from another, and even one player or band from another.

Timbre (the tone-quality of a sound) is another important distinguishing characteristic of R&B.  Most styles of R&B rely extensively on timbre variation over the course of a performance to achieve interest.  R&B singers and instrumentalists often alternate between gentle, smooth timbres and harsh, raspy ones, giving the music a wide range of emotional expression.