[jcranmer]

I'm not an expert in Fortran, but my understanding is that Fortran 90 is basically the equivalent of C++11--it added a slew of major features (such as free format code and array notation) that makes it a pretty different language from pre-90 code. Even if newer language revisions add some more useful features, it's the core set from Fortran 90 that's worth differentiating, much as I might describe modern C++ code as C++11 even if the project requires C++14 or C++17.

[ch_123]

This is absolutely correct - Fortran standards after 90/95 have mostly added extra features, rather than fundamental changes to how people write Fortran. Fortran 2003 added OO support, but I don’t believe that has seen widespread adoption.

[ryl00]

Fortran 2003 also added BIND(C) and ISO_C_BINDING, which are massively useful in standardizing the C interop.

[madhadron]

Oh, that's a nice thing. I still remember carefully appending underscores to names to thing to make C and FORTRAN talk in gcc/g77.