II.

Characteristics

Since its beginnings jazz has branched out into so many styles that no single description fits all of them accurately. A few generalizations can be made, however, bearing in mind that for all of them, exceptions can be cited.

Performers of jazz improvise within the conventions of their chosen style. Typically, the improvisation is accompanied by the repeated chord progression of a popular song or an original composition. Instrumentalists emulate black vocal styles, including the use of glissandi (sliding movements that smoothly change the pitch), nuances of pitch (including blue notes, the “bent” notes that are played or sung slightly lower than the major scale), and tonal effects such as growls and wails.

In striving to develop a personal sound, or tone color (an idiosyncratic sense of rhythm and form and an individual style of execution), performers create rhythms characterized by constant syncopation (the placing of accents in unexpected places, usually on the weaker beat) and by swing. Swing can be defined as a sensation of momentum in which a melody is alternately heard together with, then slightly at variance with, the regular beat. Written scores, if present, are often used merely as guides, providing structure within which improvisation occurs. The typical instrumentation begins with a rhythm section consisting of piano, string bass, drums, and optional guitar, to which may be added any number of wind instruments. In big bands the wind instruments are grouped into three sections: saxophones, trombones, and trumpets.

Although exceptions occur in some styles, most jazz is based on the principle that an infinite number of melodies can fit the chord progressions of any song. The musician improvises new melodies that fit the chord progression, which is repeated again and again as each soloist is featured, for as many choruses as desired.

Although pieces with many different formal patterns are used for jazz improvisation, two formal patterns in particular are frequently found in songs used for jazz. One is the AABA form of popular-song choruses, which typically consists of 32 measures in ¹ meter, divided into four 8-measure sections: section A, a repetition of section A, section B (the “bridge” or “release,” often beginning in a new key), and a repetition of section A. The second form, with roots deep in African American folk music, is the 12-bar blues form. Unlike the 32-bar AABA form, blues songs have a fairly standardized chord progression.