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.