When you want to create a datatype in C or FORTRAN, you do so by declaring the datatype before executing any statements. Your declarations are read by the compiler which sets up storage for your datatype. In contrast, MPI derived datatypes are created at run-time through calls to MPI library routines. Since MPI derived datatypes are often used to send or receive C or FORTRAN datatypes, in the typical scenario, you first declare your C or FORTRAN datatypes. Later, in the execution part of your program between calls to MPI_INIT and MPI_FINALIZE, you create and use your MPI derived datatypes.