/sys$common/syshlp/helplib.hlb FORTRAN, Data, Constants, REAL_8  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  A REAL*8 constant can be a basic real constant or an integer
  constant followed by a decimal exponent.  A decimal exponent has
  the form:

     Dsnn         s is an optional sign
                  nn is a string of decimal digits

  There are two implementations of the REAL*8 constant:  D_floating
  and G_floating.  G_floating requires the /G_FLOATING command
  qualifier.