In some areas of music, such as classical music, when a composer writes a song, it usually defines, in great detail, exactly how it should sound. When interpreting the composition, the idea and the initial spirit of the work will be kept to the fullest and no note of what is written in the score will be changed.
But even in this area where music is so well defined in the score, changes are occasionally made; for example, a score written for a symphony orchestra can be adapted in order to be played by a violin and piano duo.
In the field of jazz, modifications of the original score are not only frequent, but they are the norm. The reasons that lead to modifying the original version are multiple and they are not solely due to the character of improvisation that accompanies that particular style. Perhaps the main cause of the existence of versions is the fact that there is no score from the original song or, if it exists, are often not as accurate as in classical music. In the 20-40s, when a score publisher decided to publish a song that had become popular and wanted to sell written copies, he was already adapting the song. Each publisher did his and, often, did not agree with the one of the others. In addition, each one of these scores only included a small part of the information of the song that played the band. They usually included the melody, the lyrics (if it was a sung song) and the piano accompaniment. The accompaniment often included some countermelodies or the bass line. Some scores also included guitar chords but little else. When playing it, the musicians had to complement it by deciding what each instrument would do, creating their own version of the song.
Another reason for the modifications is the need to adjust the piece to the characteristics or musical style of the moment (swing in our case), to give it a different functionality (adapting it to dance, for example), to fit it in the limited duration of a disc, or, simply, to differentiate itself from how other bands played it.
This adaptation is called versions or arrangements and the person who writes them is the arranger. He determines how the piece will sound and what each musician will do at any time. The arrangement can be written, determined during rehearsals, or it can even be decided while performing. Normally, the bigger the group the more accurate the arrangement. This arrangement can (or should) include some degree of improvisation or changes that are made on the fly, both from oral orders and through the indications made by the leader of the band.
Groups that play for dancers very often improvise choruses. In this case, the leader of the band usually indicates who will play the next solo, how long it will last, and what the other instruments will do during these improvisations. The following video is a good example of this:
Obviously, each new version of the tune contains the main elements which allow you to identify the song. We are talking about the harmonic base (the chords that make up the piece) and the melody. There are compilations (formerly books, but nowadays applications are available) where you can find the structure, with its chords, of the majority of jazz standards (you can see a detailed explanation of what is a standard on the page that we have created on this subject). The harmony of each section is defined as is the structure that follows the sections in the song (for example AABA or 12-bar blues).
Therefore, what changes is the number and order of the sections and non-essential elements (such as introduction, interludes and the outro). Changes also usually take place in the tempo of the piece and the ornaments played by the instruments or accompaniments. This means that the same song interpreted by different bands may sound completely different. Needless to say, in principle, the solos contained in the song (where the musicians improvise what they play on the fly) will also be different.
The following video shows three versions of Jive at Five, a swing classic (and a jazz standard too), recorded for the first time by Count Basie in 1939.
If you are still interested, the next page includes different versions of Rose Room. Note how the arrangements not only change the structure of the song, but also the characteristics of the instrumentation and the interpretation. You can also go to the page where we talk about what jazz standards are and where they originated.
Rose Room, example of a song versions.
What are jazz standards and swing classics?