Each new standard usually removes a very small number of features, but Fortran 2018 is mostly a superset of Fortran 77. Most compilers retain absolute backward

Each new standard usually removes a very small number of features, but Fortran 2018 is mostly a superset of Fortran 77. Most compilers retain absolute backward compatibility, so they can compile a superset of Fortran 66 to 2018, and often some custom language extensions on top.