The Fortran 90 KIND mechanism is a standardization, regularization, and generalization of the common ``*size" extension to Fortran 77. For example, though they are not standard, it is common for REAL*4 to be the same as single precision REAL and REAL*8 to be equivalent to DOUBLE PRECISION; the ``*size" syntax is a form of parameterization of the different kinds of real data types. Fortran 90 formalizes this concept of a type kind and associates an integer kind value with each intrinsic data type. These kind values are left implementation dependent (more about that in a minute), but if an implementation chooses the value 4 for single precision REAL and 8 for DOUBLE PRECISION, then alternative ways of specifying REAL are REAL(4) and REAL(KIND=4), and alternative ways of specifying DOUBLE PRECISION are REAL(8) and REAL(KIND=8). The complete formal syntax for the REAL type specifier in declaration statements is therefore:

REAL [([KIND= ] kind-value )]

Though some implementations may choose kind values 4 for single precision real and 8 for double precision real, so that these numbers are similar to the common ``*4" and ``*8" extensions, or to represent the number of bytes in an object of this kind (the original motivation for the 4 and 8), this is not appropriate for all architectures. Some machines, for example (e.g., the Cray vector supercomputers), use a different (from 4) number of bytes for single precision real objects. Possibly more important, an implementation might support more than one form of single precision, one the default REAL and the other(s) using the same amount of storage but a different representation method. For example, on a machine whose native arithmetic is not IEEE, a Fortran implementation might support a second form of (single precision) real based upon IEEE arithmetic. In Fortran 90 terms, this would merely be another kind of real, with a different type kind value. There is therefore no ``obvious" set of kind values optimal for every implementation, which is the reason the kind values are left implementation dependent.