a - a - b - a
with 'a' being more or less the same musical phrase each time, while 'b' uses a different, contrasting melody and a different chord progression. The 'b' section is called 'the middle eight' or sometimes 'the release' or 'the bridge'. Think of Ice Cream or One Sweet Letter from You as examples.
This 32-bar routine comes very naturally to musicians and audiences because it fits in with our instinctive toe-tapping or hand-clapping patterns and also because the triple appearance of the 'a' section reinforces our perception of the melody and makes it easy to pick up.
But just occasionally you come across a tune that FEELS like a 32-bar a - a - b - a structure but in fact throws in a surprise. I recently noticed a couple.
These Foolish Things fits the pattern except that the Middle Eight is actually a MIDDLE FOUR. (I am referring to the sheet music version. I note that jazz groups sometimes make the four bars into eight by playing all the notes at double the written length.) So the tune (in its correct form, at least) ends up with 28 bars instead of 32. Try it and you will see what I mean.
What Am I Living For? (made famous by the Kid Thomas Band) clearly has a Middle Four - not a Middle Eight.
Even more surprising is Top Hat, White Tie and Tails, where the Middle Eight becomes a Middle TEN! How does this happen? Well, at the end of the first four bars of the 'Middle Eight', the long note in the fourth bar is extended for another full bar. Then this happens again after four more bars. It sounds weird? But watch Fred Astaire dancing and singing (on You Tube) and you will see for yourself that this is exactly what happens. It's as if the extra two bars (which have the effect almost of pauses) give a chance for the performer to catch a breath.
The book Playing Traditional Jazz by Pops Coffee is available from Amazon.