Most jazz since the bebop era is based on a form that is actually quite similar to the sonata allegro form from classical theory: an optional introduction, the exposition or theme (possibly repeated), the development section, and the recapitulation, possibly followed by a coda. The introduction, if present, sets the tone for the piece; the exposition is the main melody; the development section is where the composer extends the ideas of the exposition; the recapitulation is a restatement of the theme; and the coda is an ending. In jazz terms, these sections of a piece would be called the the intro, the head (possibly repeated), the solo section, the head out, and possibly a coda or tag ending. The intro establishes the mood; the head is the main melody; the solo section is where the soloists improvise on the melody and/orchord progression of the tune; the head out is a restatement of the theme; and the coda or tag is an ending.There are two very common forms for a head or theme in jazz. The first is the blues form, which is normally a twelve bar form. There are many variants on blues chord progressions, but most are based on the idea of three four bar phrases. In its original form, the second phrase would be a repeat of the first, and the third would be an answer to that phrase, although this convention is rarely adhered to in jazz.The other common form in jazz is the AABA song form, used extensively in popular music from the turn of the century until the dawn of rock and roll. This form consists two sections, called the verse or A-section and the bridge. The form is verse 1, verse 2, bridge, verse 3. The verses are similar or identical except for the lyrics and perhaps the last two bars. The song "I Got Rhythm" by George Gershwin, is one example of an AABA formIn the key of C, you might replace the G7 chord with a G7#11, a G7alt, a G7b9b5, or a G7+ chord, all of which still fulfill the dominant function in C but imply different scales. For instance, if you choose G7#11, the progression then becomes D dorian, G lydian dominant, C major.