Fortspinnung

Fortspinnung () is a German term conceived in 1915 to refer to a specific process of development of a musical motif. In this process, the motif is developed into an entire musical structure by using sequences, intervallic changes or simple repetitions. As a principle it exposes a thematic capus (German: "Themenkopf") and from there on opens itself to a (mostly) counterpuntal structure, while the reentry of same "Themenkopf" (often transposed) diverts from the fact, that there is no actual closing formula in the "Fortspinnungsthema". Thus the "Forspinnungsprinzip" counters the principle of the German "Periode", common to the "Wiener Klassik" sonata, which is metrically finite (albeit not closing, as such): Everything in the "Periode" points to a metrically defined point, where its 'end', its 'right border' is to expect. This pointing structure is missing, i. e. carefully averted in the "Fortspinnung". Thus the "Gestalt" of the "Fortspinnungsthema" has a 'rag margin', while the "Periode" ends more predictably.
It is also possible to use the "fortspinnung form" term that stands for a structure with three parts: 
In Baroque music, melodies and their lyrics were prose. Rather than paired lines they consist of rhetorical sentences or paragraphs consisting of an opening gesture, an amplification (often featuring sequence), and a close (featuring a cadence); in German "Vordersatz"-"Fortspinnung"-"Epilog". For example:
 When I was a child, [opening gesture]
 I spake as a child, [amplification...]
 I understood as a child, [...]
 I thought as a child; [...]
 But when I became a man, I put away childish things. [close]
 —1 Corinthians 13
Examples.
The use of this process of development and musical structure is focused mainly in the Baroque period. Here is an example of the "fortspinnung" process in the first section of the Johann Sebastian Bach Invention No. 4 in D minor (BWV 775):
Even though it is a Baroque technique, it is also possible to find this process in scores from other composers in different periods, such as Johannes Brahms in the Romantic period: