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- FORTRAN 66 comments are denoted by the 'C' character in column 1, while FORTRAN 77 comments may also use '*' i column 1 instead. Fortran 90 also allows the use of '!' character.
- FORTRAN 77 symbolic names are limited to 6 character lengths, while Fortran 90 allows names up to 31 characters long.
- FORTRAN 77 files need 6 spaces before words begin, while Fortran 90 doesn't (as it uses the free-form code style.)