http://gcc.gnu.org/wiki/GFortranGettingStarted
Free-form, fixed-form and file extensions
By default, gfortran is aware of a few file extensions, and its action depends on the extension of the file you ask it to compile. Examples:
- if the file is named code.f gfortran supposes it is fixed-form source, with no preprocessing needed.
- if the file is named code.f90 gfortran supposes it is free-form source, with no preprocessing needed.
- if the file is named code.F gfortran supposes it is fixed-form source and performs preprocessing.
- if the file is named code.F90 gfortran supposes it is free-form source and preprocess it.
There are other extensions recognized, such as: .f95 is the same as .f90 and .F95 is the same as .F90
Of course, you can override gfortran’s default, with options -ffree-form and -ffixed-form (many gfortran options are named -f''=option-name ‘. For example, if you have a free-form code named bessel.f, you can compile it with
gfortran -ffree-form bessel.f -o bessel.exe