This relates to the manner in which all the sections (or melodies if more than one) are structured in what particular order. Again, the purpose is to create a satisfyingly integrated whole. For example, in classical music, the “gold-standard” of structural form was Sonata-Allegro. In the widest sense this consisted of an Exposition (statement of themes 1 & 2, or more), Development (variation on themes), Recapitulation (restatement of themes 1 & 2, more). The form was highly plastic, though, allowing many different possibilities for variation & alteration. There were other forms: Rondo, Minuet & Trio, etc. Larger forms were the symphony, the concerto, etc.

There are some modern day musicians who claim to “hate the rules” of all that “stuffy classical” music. First, it is ironic because any standard pop song’s structure with verse & chorus, optional bridge/solo, repeat verse & chorus exactly mimics sonata-allegro form on a smaller scale.  Next, the richness in integrated components (themes, harmonies, rhythms, etc.) of the simplest Haydn piano sonata (for example) is well beyond anything any pop song has ever achieved. Don’t get me wrong here. There is some pop music which I think has great value; there is some pop music I personally enjoy. I am not saying it is bad. Some of it is good, some brilliant, some genius.  By it’s nature, I think it has to be more brief & to the point (i.e. use less musical tools & info available to a composer). Also, the biggest area of focus for variety in pop music over the last 100 years is in timbre.  The form & structure of any given early 1920s jazz song is comparable to a pop song from the 1950s, 1960s or even the 2000s.  The primary difference is in the timbre of the instruments.

Finally, a great deal more variety in form & structure (along with melody, harmony & rhythm) has been achieved in classical music compared to pop music.  There’s more structural form variation & integration in ONE Beethoven sonata than in the last 50 years of pop music.