Novels, plays, films and longer short stories will have plots and sub-plots. Andrew Melrose identifies a pyramid. The main plot takes up most of the room and the smallest plot the least of the text. Pile these plots one upon another – we get a pyramid. There is a strict relationship between the size of the plots. Possibly it relates to the Golden Segment and the Fibonacci series. It may be a learnt response; even if it is, it is what we are used to and what we enjoy and expect.
I add to Melrose's proposal. I argue that subplots are not extra plots but that they are part of the main plot. Thus we get in Cinderella:
Main Plot = transformation
Plot 2 = battle with Ugly Sisters and Stepmother
Plot 3 = Can she get to the ball? (aka her rightful place in society)
Plot 4 = Cinders and the Prince
Plot 5 = The Prince and the shoe
Plot 6 = The shoe and the foot
Plot 6 is the small one at the top.
All plots resolve at the same moment. As the shoe fits on to Cinders' foot, the Prince is rewarded for persevering. Cinders' relationship with the Prince is confirmed. Cinders takes up her rightful place in society. She wins her battle with her stepsisters and her life is transformed.
Note also how that important final small sub-plot contains that all important crisis point; if the shoe does not find its way to the foot, life will be forever different for both Cinders and the Prince. There's a good argument here also for having one last nasty thing happen before it all resolves. This heightens the tension. So, sometimes Cinders is hidden in a cupboard. In more gruesome versions the sisters chop off toes.
Creative reading exercise:
Take a novel you've enjoyed and work out what are its sub-plots. Do they relate to each other to form a pyramid? Do they all resolve at the same time? Does the smallest one include the crisis point?
Creative writing exercise
Look at the plan of your latest story. Does it contain balanced sub-plots? If not, can you put them in? Can they all resolve at the same time? Does the smallest contain the crisis point?
Use the sub-plot / plot theory to see whether your text has balance. Possibly if it seems out of kilter something is not quite right here. Try balancing these plots / sub-plots in a pyramid. Remember to have all the drama in the smallest sub-plot and let them all resolve at the same time.