Original: By 1965, Fortran IV was supposed to be the "standard" and in compliance with American Standards Association X3 .4.3 FORTRAN Working Group.
Simplified: Several revisions of the language have appeared, including the very well known FORTRAN IV (the same as FORTRAN 66), FORTRAN 77 and Fortran 90. The most recent formal standard for the language was published in 1997 and is known as Fortran 95.