Most jazz historians begin with an attempt to define the subject, and just as you think you have stumbled on an author who has finally got it right, you realise that there are so many exceptions that the exercise is futile. There is no general agreement on the derivation of the term and even jazz singers find it very hard to define the word except in terms of itself (‘I am a jazz singer. I sing jazz. I am a jazz singer.’). Jazz means a certain sort of repertoire: the historical accretion of songs and tunes over the course of the twentieth century has created a body of musical material that is categorised loosely as jazz. Jazz is also a particular set of expressive skills and styles that are brought to bear on this repertoire, a feeling and a freedom to create as well as simply to interpret. A significant element of this is, or may be, improvisation. Although for many improvisation is a defining characteristic of the music, it is perfectly possible to have a kind of jazz without it. Frank Sinatra and Ella Fitzgerald, among others, sang and recorded swathes of what has come to be called the Great American Songbook, often faithfully reproducing the composers' and arrangers' scores with little or no improvisation. Swing is another element that many would consider crucial to most jazz, yet some singers found under ‘jazz’ in record shops (Astrud Gilberto comes to mind) are not usually associated with the term.