Fortran Wiki
implicit none

In Fortran, by default the type of variables whose names start with I...N is INTEGER, and REAL otherwise. It is a legacy of the past and it is know strongly advised to put an implicit none statement in your program and in each module. That statement was added in the Fortran 90 standard.

Be conscious that if you put an implicit none at the top of a module, it will apply to the procedures in the contains section of the module, but not to the interface section:

module example
implicit none

interface
  integer function one(i)
    ! Here i will be implicitly an INTEGER, except if you add an implicit none here
  end function
end interface

contains

  integer function two(j)
    ! The compiler will detect an error because j is not declared
  end function jfun

end module example

See also:

• Doctor Fortran in “IMPLICIT Dissent”
• Where to put implicit none in Fortran (stackoverflow)