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

     /PRECISION={SINGLE | DOUBLE}

  Controls whether floating-point operations on float variables are
  performed in single or double precision.

  The default is /PRECISION=DOUBLE for /STANDARD=VAXC and
  /STANDARD=COMMON modes.

  The default is /PRECISION=SINGLE for /STANDARD=ANSI89 and
  /STANDARD=RELAXED_ANSI89 modes.