The declaration can be: DOUBLE PRECISION or REAL*8.
For a declaration such as DOUBLE PRECISION X, the variable X is a REAL*8 element in memory, interpreted as one double-width real number.
If you do not specify the size, a default size is used. The default size, for a declaration such as DOUBLE PRECISION X, can be altered by compiling with any of the options -dbl, -r8, or -xtypemap. See the discussion in Chapter 2 for details.
For a declaration such as REAL*8 X, the variable X is always an element of type REAL*8 in memory, interpreted as a double-width real number.