Real

 A real constant approximates the value of a mathematical real
 number.  The value of the constant can be positive, zero, or
 negative.

 VSI Fortran provides three kind type parameters for data of type
 real:  REAL(KIND=4) (or REAL*4), REAL(KIND=8) (or REAL*8), and
 REAL(KIND=16) (or REAL*16).  REAL(KIND=8) is DOUBLE PRECISION.  If
 DOUBLE PRECISION is used, a kind type parameter must not be
 specified for the constant.

 The type specifier for the real (single-precision) type is REAL;
 the type specifier for the double precision type is DOUBLE
 PRECISION.

 If a kind type parameter is specified, the real constant has the
 kind specified.  If a kind type parameter is not specified, the
 kind is default real.

 The following is the general form of a real constant with no
 exponent part:

   [s]n[n...][_k]

 A real constant with an exponent part has one of the following
 forms:

   [s]n[n...]E[s]nn...[_k]
   [s]n[n...]D[s]nn...
   [s]n[n...]Q[s]nn...

    s   Is a sign; required if negative (-), optional if 
        positive (+).

    n   Is a decimal digit (0 through 9). A decimal point 
        must appear if the real constant has no exponent part.

    k   Is an optional kind type parameter (4 for REAL(KIND=4), 
        8 for REAL(KIND=8), or 16 for REAL(KIND=16)).  It must 
        be preceded by an underscore (_).

 Leading zeros (zeros to the left of the first nonzero digit) are
 ignored in counting significant digits.  For example, in the
 constant 0.00001234567, all of the nonzero digits, and none of the
 zeros, are significant.  (See the following sections for the number
 of significant digits each kind type parameter typically has).

 The exponent represents a power of 10 by which the preceding real
 or integer constant is to be multiplied (for example, 1.0E6
 represents the value 1.0 * 10**6).

 A real constant with no exponent part is (by default) a
 single-precision (REAL(KIND=4)) constant.  You can change this
 default behavior by specifying the compiler option
 /ASSUME=FPCONSTANT.

 If the real constant has no exponent part, a decimal point must
 appear in the string (anywhere before the optional kind type
 parameter).  If there is an exponent part, a decimal point is
 optional in the string preceding the exponent part; the exponent
 part must not contain a decimal point.

 The exponent letter E denotes a single-precision real (REAL(KIND=4)
 or REAL*4) constant, unless the optional kind type parameter
 specifies otherwise.  For example, -9.E2_8 is a double-precision
 constant (which can also be written as -9.D2).

 The exponent letter D denotes a double-precision real (REAL(KIND=8)
 or REAL*8) constant.

 The exponent letter Q denotes a quad-precision real (REAL(KIND=16)
 or REAL*16) constant.  A minus sign must appear before a negative
 real constant; a plus sign is optional before a positive constant.
 Similarly, a minus sign must appear between the exponent letter (E,
 D, or Q) and a negative exponent, whereas a plus sign is optional
 between the exponent letter and a positive exponent.

 If the real constant includes an exponent letter, the exponent
 field cannot be omitted, but it can be zero.

 To specify a real constant using both an exponent letter and a kind
 type parameter, the exponent letter must be E, and the kind type
 parameter must follow the exponent part.

Additional information available:

DOUBLE_PRECISION                REAL_4          REAL_8          REAL_16