Developmental material is a catch-all term used to describe any part of a song that doesn’t clearly fit into a defined section. Usually, these sections consist of material that has already been introduced, but has been transformed in some way; however, they can, less commonly, be used to introduce new material.
That makes sense, but what does “transformed in some way” actually mean?
The best way to illustrate how a composer can develop, or transform, melodic and harmonic content is through score study. Let’s take a look at Invention no. 8 by J.S. Bach.
Motif 1 is an ascending arpeggio pattern that ends an octave higher than it starts. Notice how subsequent statements will differ slightly, but still follow a similar pattern.
Motif 2 is descending scale pattern that serves as a response to motif 1. Bach uses this motif in sequence to move to the dominant key.
Motif 3 is purely sequential material, that is a motivic idea that’s primary function is to be repeated at different transpositions. Sequences are commonly used to tonicize or modulate closely related keys.
Motif 4 acts as a bridge between statements of motifs 1 and 2. Bach uses this motif to obscure the key and as a means to switch motifs 1 and 2 between the hands.
The areas labeled “filler” material could possibly be counted as a fifth motif, however, the lack of motivic development of these sections, the strict transposition and repetition, and their proximity to the two primary cadences of the work point more towards “free” counterpoint (a common practice in fugues and other contrapuntal works, used to fill in space between thematic statements).