IMPLICIT Statement

  The IMPLICIT statement can be used to change the first-letter default rule throughout a program unit. For example:
IMPLICIT DOUBLE PRECISION (D,X-Z), INTEGER (N-P)
would mean that all names starting with the letters D,X,Y, or Z would (unless declared otherwise in type statements) have the type double precision. Similarly the letters I through P, instead of just I through N, will imply integer type. The other letters (A-C,E-H, and Q-W) will still imply real type.

IMPLICIT can be used with character type to specify a default length as well, for example:
IMPLICIT CHARACTER*100 (C,Z), CHARACTER*4 (S)
But this is not usually of much practical value. As with type statements, the default character length is one.

More than one IMPLICIT statement can be used in a program unit but the same letter must not have its implied type specified more than once. The usual Fortran implied-type rules apply to all initial letters not listed in any IMPLICIT statements. The list of letters given after each type must appear in alphabetical order. IMPLICIT statements normally precede all other specification statements in a program. There is one exception to this: PARAMETER statements may precede them provided that the constants named in them are not affected by the IMPLICIT statement. Note that dummy arguments and function names may be affected by a subsequent IMPLICIT statement. IMPLICIT statements have no effect on intrinsic function names.