During the golden age of the American popular song (around 1915-60), several dozen very talented composers wrote a countless number of flexible songs that were adopted (and often transformed) by creative jazz musicians and singers. Often originally written for Broadway shows and Hollywood films, many of these works (generally 32 bars in length) have been performed and recorded a seemingly infinite number of times, including "Body and Soul," "Stardust," and "All the Things You Are." Such composers as Jerome Kern, Irving Berlin, George Gershwin, Harold Arlen, Hoagy Carmichael, Cole Porter, Richard Rodgers, Harry Warren, Fats Waller, and Duke Ellington supplied the jazz and pop music worlds with what must have seemed like an endless supply of gems. Called Standards (which means that they caught on as a permanent part of the jazz and pop music repertoire), the songs differ from less flexible "originals" that are often put together for a record date and then quickly forgotten. Since the rise of rock, the pop music world has been a much less fertile area for jazz players to "borrow" material from, and although many of the old standards are still performed, jazz musicians and singers have had to rely much more on original material since the 1960s. — Scott Yanow

Modal Music is not so much a style of jazz as it is a structure. Before the advent of modal music in the '50s, solo improvisations were based around the specific key of a piece — that is, its tonal center, the starting point to which its melodies and chord progressions would return for a feeling of resolution or completeness. Modal improvisations, on the other hand, were based on modes, or scales — but not just the typical major and minor scales familiar to nearly all musicians. The most commonly used modes did relate to major scales, though; each note in the scale was also the first note of a new mode, which would incorporate all of the notes in the original major scale, but sounded different because the new starting point rearranged the order of distances between notes. Thus, if a musician were improvising over, say, a D chord, he could essentially choose any key center whose corresponding scale included the note D (unless the composer dictated specific modes to be used in the solos). The new order of distances between notes could produce very different moods — for example, even though the C major and A minor scales use exactly the same notes, the former sounds bright and happy, whereas the latter is more melancholy. Because the feeling of the music came from those note choices, chord progressions in modal music were usually kept very simple, since too much motion would have allowed little time to fully explore the mode(s) that had been selected. The results often had a meditative, cerebral feel, but weren't quite as mellow as, for example, West Coast-style cool jazz. Modal music had a subtle tension produced by the fact that the solo lines, while melodic, didn't always progress or resolve exactly as the listener was accustomed to hearing; plus, every time a new mode was introduced, the tonal center shifted, keeping the listener just off balance with a subtle unpredictability. Miles Davis was the first jazz musician to improvise and compose according to modal structure; his Kind of Blue is the definitive modal jazz album, and two of his sidemen on the record — John Coltrane and Bill Evans — later went on to become modal innovators in their own right. The freedoms of modal music helped pave the way for the rabid structural experimentation of avant-garde jazz, which would begin to take shape toward the end of the '50s.

Although some history books claim that Hard Bop arose as a reaction to the softer sounds featured in cool jazz, it was actually an extension of bop that largely ignored West Coast jazz. The main differences between hard bop and bop are that the melodies tend to be simpler and often more "soulful"; the rhythm section is usually looser, with the bassist not as tightly confined to playing four-beats-to-the-bar as in bop; a gospel influence is felt in some of the music; and quite often, the saxophonists and pianists sound as if they were quite familiar with early rhythm & blues. Since the prime time period of hard bop (1955-70) was a decade later than bop, these differences were a logical evolution and one can think of hard bop as bop of the '50s and '60s. By the second half of the 1960s, the influence of the avant garde was being felt and some of the more adventurous performances of the hard bop stylists (such as Jackie McLean and Lee Morgan) fell somewhere between the two styles. With the rise of fusion and the sale of Blue Note (hard bop's top label) in the late '60s, the style fell on hard times although it was revived to a certain extent in the 1980s. Much of the music performed by the so-called Young Lions during the latter decade (due to other influences altering their style) was considered modern mainstream, although some groups (such as the Harper Brothers and T.S. Monk's sextet) have kept the 1960s' idiom alive. — Scott Yanow

It has become increasingly difficult to categorize modern jazz. A large segment of the music does not fit into any historical style; it is not as rock-oriented as fusion or as free as avant garde. Starting with the rise of Wynton Marsalis in 1979, a whole generation of younger players chose to play an updated variety of hard bop that was also influenced by the mid-'60s' Miles Davis Quintet and aspects of free jazz. Since this music (which often features complex chordal improvisation) has become the norm for jazz in the 1990s, the terms modern mainstream or Post-Bop are used for everything from Wallace Roney to John Scofield, and symbolize the eclectic scene as jazz enters its second century. -- Scott Yanow