VMS Help FORTRAN, Statements, COMMON  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Defines one or more contiguous blocks of storage shared among
  separate subprograms.  You can define the same common block in
  different program units of your program.  The first COMMON
  statement in a program unit to name a common block defines it;
  later COMMON statements that name the block reference it.  You can
  leave one common block (the "blank" common block) unnamed.
  Statement format:

     COMMON [/[cb]/]nlist[[,]/[cb]/nlist]...

     cb     Is a symbolic name to identify the common block.

     nlist  Is one or more names of variables, arrays, array
            declarators, or records to identify elements of
            the common block.

  Any common block name, blank or otherwise, can appear more than
  once in one or more COMMON statements in a program unit.  The list
  following each successive appearance of the same common block name
  is treated as a continuation of the list for the block associated
  with that name.

  You can use array declarators in the COMMON statement to define
  arrays.

  A common block can have the same name as a variable, array, record,
  structure, or field.  However, in a program with one or more
  program units, a common block cannot have the same name as a
  function, subroutine, or entry name in the executable program.

  When common blocks from different program units have the same name,
  they share the same storage area when the units are combined into
  an executable program.

  Entities are assigned storage in common blocks on a one-for-one
  basis.  Thus, the entities assigned by a COMMON statement in one
  program unit should agree with the data type of entities placed in
  a common block by another program unit; for example, consider a
  program unit containing the following statement:

     COMMON CENTS

  Consider another program unit containing the following statements:

     INTEGER*2 MONEY
     COMMON MONEY

  When these program units are combined into an executable program,
  incorrect results can occur if the 2-byte integer variable MONEY is
  made to correspond to the lower-addressed two bytes of the real
  variable CENTS.