A REAL(8) or DOUBLE PRECISION constant has more than twice the accuracy of a REAL(4) number, and greater range.
A REAL(8) or DOUBLE PRECISION constant occupies eight bytes of memory. The number of digits that precede the exponent is unlimited, but typically only the leftmost 15 digits are significant.
On Tru64 UNIX, Windows NT, and Windows 9* systems, IEEE T_floating format is used. On OpenVMS systems, either Compaq VAX D_floating, G_floating, or IEEE T_floating format is used, depending on the compiler option specified.
The following examples show valid and invalid REAL(8) or DOUBLE PRECISION constants:
Valid  
123456789D+5  
123456789E+5_8 

+2.7843D00 

.522D12 

2E200_8 

2.3_8 

3.4E7_8 

Invalid  Explanation 
.25D0_
2  2 is not a valid kind for reals. 
+2.7182812846182  No D exponent designator is present; this is a valid singleprecision constant. 
1234567890D45  Too large for D_floating format; valid for G_floating and T_floating format. 
123456789.D400
 Too large for any doubleprecision format. 
123456789.D
400  Too small for any doubleprecision format. 