A jam session will provide jazz players with practice time that helps them to improve their sound and practice improvisation techniques. While not specifically a jazz improvisation technique, the jam session plays an integral role in the development of a jazz performer's ability to create music on the spot. During these sessions, the players will take turns improvising while the rest of the ensemble supports them with melody and harmony. This provides an essential practice period where performers try out new ideas and develop improvisational skills.
The pentatonic scale makes it possible to quickly improvise melodic ideas on top of a roaming bass line. The performer will play the scale based on the key and chord changes that appear in the music. For instance, if the music calls for a B flat major chord, then the jazz performer can improvise on any pentatonic scale that includes a B flat. The pentatonic scale consists of five notes. Technically, any five notes may be used as long as they span the distance of one octave.
Jazz musicians will often use uncommon scales to improvise with notes outside of the regular major and minor scales. The blues scale can be thought of as a mix between a major, minor and diminished scale. The blues scale has all of the diatonic notes from C to C, but it also includes an optional flat third, fifth and seventh scale degree. This allows the jazz performer to improvise in a regular major key. For instance, in C major, the jazz musician knows that he can use any notes of the C major scale, but can also throw in an E flat, G flat or B flat to add some extra tension and flavor to the music.
Chords are typically used for harmony, but they also allow the jazz improviser to create advanced, vaguely related melodies that are still based in the key. For instance, when playing in the key of C major, the jazz player is given a C major chord to improvise on. She can start by playing the notes C-E-G, which are in the chord of C major. Then, she can go outside the key, by using the E of the C major chord to build another chord, E-G-sharp-B. This allows her to leave the key of C major in a way that creates a smooth transition from C major to E major.