"Rhapsody in Blue" features both rhythmic invention and melodic inspiration, and demonstrates Gershwin's ability to write a piece with large-scale harmonic and melodic structure. The piece is characterized by strong motivic interrelatedness. Much of the motivic material is introduced in the first 14 measures. Musicologist David Schiff has identified five major themes plus a sixth "tag". Of these, two appear in the first 14 measures, and the tag shows up in measure 19. Two of the remaining three themes are rhythmically related to the very first theme in measure 2, which is sometimes called the Glissando theme—after the opening glissando in the clarinet solo—or the Ritornello theme. The remaining theme is the Train theme, which is the first to appear at rehearsal 9 after the opening material. All of these themes rely on the blues scale, which includes lowered sevenths and a mixture of major and minor thirds. Each theme appears both in orchestrated form and as a piano solo. There are considerable differences in the style of presentation of each theme.