Many songs begin and end with identical, or very similar, sections. Often there will be a distinctly different section in the middle, which may be called the “Bridge”. The most common song form, AABA, has a melody line (the A section), which is repeated. After playing the A section twice, it is on to the bridge (the B section), then finally another recitation of the A section, or something very similar.
Bridges are often quite different from the A section, in order to add interest to the song. This creates an additional challenge for the improviser because the bridge is often in a different key center altogether.
Take a few minutes to become common with the Key - Chord Relationships chart. Now study the first tune below, the Tech Stomp. If you are looking at the concert-key version of this song, notice that every chord throughout this song can be found in the first row of the Key-Chord chart. In other words, The Tech Stomp remains in the key center of C 100% of the time. This means that the improviser can safely use the notes from the C major scale throughout the song.
The other songs below move among different key centers during the song. When this happens, you need to find the new key center and improvise using notes from that key center.