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

  /[NO]D_LINES                    D=/NOD_LINES

  Controls whether the compiler reads and compiles lines that have a
  "D" in column 1 of the source program.  If you specify /D_LINES,
  lines that have a "D" in column 1 are compiled.

  The default is /NOD_LINES, which means the compiler assumes that
  lines beginning with a "D" are comments and does not compile them.