Hi All,  
  
I have trouble with writing larger/longer pieces of music. 
I can come up with melodies or "riffs", but I'm not sure how 
to make a piece "coherent" and not simply a string of ideas, 
or a "song", or ABA form. 
  
To help me understand, I would like to start with a very simple 
and short example, prelude in C by Bach, BWv924 (see attached). 
Why is this piece "coherent"? What is the structure or 
form of this piece? 
  
Thanks for any help 
IMSLP222728-PMLP180599-Bach_Prelude_BWV924_Cmaj.pdf