Let’s take a look at three terms that are often used when people talk about jazz improvisation: a “standard”, a “chorus” and the “head” or “head arrangement”.
What do jazz musicians improvise on/over?
They usually use a song, and improvise over its chord structure. Very often these are so called “standards”. These standards are actually tunes from Broadway shows such as George Gershwin’s “Summertime” or “I got rhythm“. Often people don’t realise that these tunes actually originated in shows; jazz musicians just took them, played them, soloed on them, rearranged them, in short: they did their own thing with them. Over time these tunes had become so common in the world of jazz that musicians started referring to them as “standards”. You can now buy whole books full of jazz standards. These standards are usually 32 bars long. Most pop songs are 32 bars as well – this is due to the fact that most self-contained sections of a song are 8 bars long. So the first section, or A section, of a song would be 8 bars. You would then repeat those 8 bars (because they were so great the listener wants to hear them again), then play a new section (B Section), and then repeat the original A section. Hence you often hear people referring to an AABA structure, or a (8+8+8+8=) 32 bar song form.
You could also improvise over the standard 12 bar blues or an original jazz piece, such as “Cantaloupe Island” by Herbie Hancock. Cantaloupe Island happens to be 16 bars long, which is slightly unusual.
Now if you have finished one complete run through/improvisation from the first bar to the last bar of tune, you call that one “chorus”. If you’ve played through twice that would be two “choruses”, etc. There are no hard and fast rules over how many choruses one should play for a solo, but it depends on the player. John Coltrane for example didn’t know when to stop, and rumour has it that he once started playing in the taxi on the way to a gig, so I guess the audience (and the rest of the band) missed the first part.
The speed of the piece is also important. If you are playing a really slow ballad, you probably don’t want to play too may choruses, but naturally on faster pieces you end up playing more choruses. Musicians also usually play longer solos on a gig than they do on records. Drummers, when they do solo, sometimes have shorter solos, though there is also a tradition of giving the drummer just one solo for the night, and letting him have a really long one. There are many viscous rumours that most punters use this opportunity to pop to the loo (wc). Tut tut.
Once every member of the band (who wanted to improvise) has finished his or her solo, you would normally return back to playing the actual theme of the song. You call this the “head” or “head arrangement”. But on some occasions players may slot in “trading fours” (or indeed trading twos or eighths).
You may sometimes see a musician pointing to his head on a band-stand – this is the signal for everyone to return to the head of the piece.
Within a jazz performance the lead player would usually play the head first, which would be followed by the solos (of each band member who wanted to improvise) and the head to finish. So the most common form for a jazz quartet consisting of sax, piano, bass an drums playing a standard would be:
1st Solo (Sax, probably 3-6 Choruses)
2nd solo (Piano, probably 3-6 Choruses)
Option 3rd/4th Solo (If drums/bass probably often one chorus, but could be longer)
Optional trading Twos/Fours/Eighths