APPLICATION
-include
  -abc.h
  -xyz.h
  -pdq.h
-ABC
  -abc.c  /* #include "pdq.h"  to use services of module PDQ */
  -abc_appSpec.h
-XYZ
  -xyz.c /* #include "pdq.h"  to use services of module PDQ */
  -xyz_appSpec.h
-PDQ
  -pdq.c
  -pdq_appSpec.h

Example 1: Where module source files might reside in a typical simple source directory.