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;
  subsequent 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 that identifies the common block.

     nlist  Is one or more names of variables that identify items in
            the common block. The variable must not be a dummy
            argument, allocatable array, automatic object, function,
            function result, or entry to a procedure.

            It must not have the PARAMETER attribute.  If an object
            of derived type is specified, it must be a sequence type.

  A common block is a global entity, and must not have the same name
  as any other global entity in the program, such as a subroutine or
  function.

  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.

  A variable can appear in only one common block within a scoping
  unit.

  If an array is specified, it can be followed by an explicit-shape
  array specification.  The array must not have the POINTER attribute
  and each bound in the specification must be a constant
  specification expression.

  A pointer can only be associated with pointers of the same type,
  kind type parameters, and rank.

  Nonpointer variables can be associated if they are of different
  numeric type.

  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.

  Named common blocks must be declared to have the same size in each
  program unit.  Blank common can have different lengths in different
  program units.