The GNU Fortran language supports two uses of the keyword
DOUBLE to specify a specific kind of type:
DOUBLE PRECISION, equivalent to REAL(KIND=2)
DOUBLE COMPLEX, equivalent to COMPLEX(KIND=2)
Use one of the above forms where a type name is valid.
While use of this notation is popular, it doesn't scale well in a language or dialect rich in intrinsic types, as is the case for the GNU Fortran language (especially planned future versions of it).
After all, one rarely sees type names such as DOUBLE INTEGER,
QUADRUPLE REAL, or QUARTER INTEGER.
Instead, INTEGER*8, REAL*16, and INTEGER*1
often are substituted for these, respectively, even though they
do not always have the same meanings on all systems.
(And, the fact that DOUBLE REAL does not exist as such
is an inconsistency.)
Therefore, this document uses "double notation" only on occasion for the benefit of those readers who are accustomed to it.